Th crown of a barrel is the muzzle end of the bore. Literally, the transition between the bore face and the end of the barrel steel. The SKS you asked about likely has a rounded crown. A damaged crown will have dings or burrs, or may have torching/pitting around the crown. Barrel crowns are important to ensure that the bullet makes a clean, even departure from the rifling, and that the propellant gases are released cleanly. For the SKS in question, or really any rifle, there's not much reason to recrown the barrel unless it has been damaged.

Recrowning is best done on a lathe, but can be done with crowning tools and proper pilots by hand. It's far from ideal, and if you do not know what you are doing, you could damage your barrel. It is NOT, under any circumstances, a project for the inexperienced. Bluntly (they call me Nomercy for a reason), if you're asking what it means, you're NOT ready to do it yourself.

Bedding the stock, actually rather bedding the ACTION into the stock, is the process of preparing the stock inlet to receive a layer of bedding compound to ensure consistent, repeatable contact between the action and stock. Bedding is typically accompanied by free floating the barrel and installing bedding pillars in the stock action screw holes. Free floating the barrel means hogging out the barrel channel to ensure there is no contact between the barrel and stock in front of the action, in other words, leaving the barrel "free-floating" in air within the barrel channel. This relieves pressure on the barrel from the stock. It's very common in sporting weight rifles that as the barrel gets heated and expands, the pressure points between the barrel and stock will change, causing the subsequent shots to "drift" or "walk". Bedding pillars installed in the stock allow a fixed tension to be applied to the action screws by creating a rigid, fixed length. In this way, if the wood were to swell or shrink, or warp, it will have little to no effect on the action screw torque, because the screws are no longer clamping the action against the wood, they're clamping it against the pillars, which are holding onto the wood.

Bedding an SKS would be a waste of time, in my opinion, and might be relatively difficult for someone inexperienced with bedding to do properly without getting bedding compound into the internal works. Frankly, it's foolish to think about bedding an SKS stock, from the aspect that there are no action screws, but rather a pivot pin and a locking tab. Adding bedding compound without removing EXACTLY the proper stock material, and adding EXACTLY the right depth of bedding compound might leave you unable to lock up the action to the FCG, and adding anything but the EXACT dimensions would not produce the even action support you're looking for anyway (since it's a levering, fixed depth fixture, not a screw fixture). I'm sure some guys have bedded SKS's, but the amount of time it took me to type this was literally MORE time than ever should be spent thinking about bedding an SKS.

Bedding is a pretty remedial level task, but a poor bedding job is a waste of time. Hogging out the barrel channel and getting proper compound thickness, and bonding to the stock takes skill. Properly seating the pillars takes skill. The downside of a poor bedding job is low, in other words, it's takes skill to do a proper bedding job to BENEFIT your accuracy, but it's pretty hard to HURT your accuracy with a bad bedding job (unless the rifle is exceptionally accurate without bedding, or you're exceptionally terrible at bedding).

Polishing a bolt basically means exactly that. Polishing the bolt. For a blued bolt, polishing can remove, or harm the integrity of the bluing, so it isn't recommended. Polishing the bolt is 100% aesthetic. Do it when you have money to burn and time to spare, and think a polished bolt would look cool.

Polishing the traces and bolt engagement is a practical application, but takes a masterful skill to do properly. This is polishing the interfacing surfaces between the bolt and action. For what it's worth, it only really makes you feel better as you cycle the bolt, and doesn't necessarily effect the accuracy of the rifle, unless you OVER DO IT and end up "polishing" your way to a loose lock up. Inexperienced hands can RUIN a firearm by improperly "polishing" the bolt traces, and can even make a firearm unsafe.

For that SKS, if you over-polish, the actions capture of the bolt may become reduced, which could cause failure. And of course, polishing ANYTHING on an SKS is much like polishing a turd. It's NOT MEANT TO HAVE EXACTING DIMENSIONS, the bolt has a very loose travel, so polishing won't help much.

Probably not as brief as you may have wanted, but these are not tasks that: "I read 100 words on the internet and did a precision action job". These are skilled tasks that take practice, understanding, and knowledge to do well.