10 X 42 are perfect for me, Great at long and sort distances.

For example as rimjob said the first number is the "zoom" the 10 makes things 10 times closer to you. the second number is the diameter of the opposite side from what you look through. The bigger the diameter the bulkier the bino's, but also the wider field of view you have with more light being gathered. 42mm is a great median. IMO