Someone has already said that it's the muscle you use to stop peeing midstream, but I wanted to add a few other points:

• Once you've identified the muscle, you shouldn't do kegels while peeing, because this can lead to UTIs and an unhappy bladder

• When you do kegels, you should try to only engage your pelvic floor muscles; try to avoid engaging other muscles in your thighs, buttocks or abdomen. Isolating your pelvic floor muscles can be tricky at first, but it gets easier

• As well as improving aspects of sexual function, kegels also help with incontinence. If you practice them now, you'll be thankful for it as you age