hello I wants to buy my first shotgun and I do not know what it means chokes: screw-in modifled I do not know whether I can assume different chokes and if I can shoot with a slug

Welcome to the forum.
Most shotguns made now have the inside of the barrel threaded to accept choke tubes of different constrictions. This lets you adjust your gun to different types of hunting. These chokes concentrate the shot for long range shooting or allow it to spread out rapidly for close shooting. A "full" choke creates a really tight pattern, a "modified" choke allows the shot pattern to open up some for moderate ranges, and an "Improved cylinder" choke allows the pattern to spread out quickly for close range. If I were shooting slugs, I would prefer an "improved cylinder" tube. This would allow the slug to leave the barrel with very little interference.

screw in chokes means that you can interchange chokes on your shotgun you can get improved cylinder skeet modified full and xfull the modified choke is probably the most used choke out of them if you shoot game with shot use it but for slugs you want the improved cylinder

Get all your chokes in the "extended" types (not flush).