Welcome to the United Mileage Plus Forum.

It's a combination of answers. The simplified answer is yes, one can standby for an earlier flight (with certain restictions) on the same day for free; what United offers as an additional option is to confirm a seat on an earlier flight for $50.

Please see the following thread from the Forum FAQ for more information:

Everything you wanted to know about the $50 confirmed standby:
http://www.flyertalk.com/forum/showthread.php?t=509034

FlyinHawaiian, Co-Moderator
United Mileage Plus Forum