If your playing limit holdem, say $2-4, the big blind = the small bet which would be $2. The small bline is 1/2 the big blind, which would be $1. The dealer button that rotates decides who the blinds are. The person to the left of the button is the small blind and to the left of the small blind is the big blind. They have to put down "blinds" of $2 and $1 before the cards are dealt out. It is like an ante but only 2 people have to do it, and that money is in play. For example the first person to act after the blinds has to fold, call the $2 blind or raise to $4. If someone raises to $4, the big blind only need to pay $2 to see the flop.
hope this helps