"Checking" a box on a form means putting a mark in it. The mark looks like ✓. Checking a box means that you're saying "yes" to whatever is written next to the box.

For example, on a form you may be asked to answer whether you are "Male" or "Female". You would "check 'Male'" if you are a man, and "check 'Female'" if you are a woman.

On online forms, clicking a box on a form usually causes the box to be checked.