The small blind is position at the poker table with a seat directly right of the button, which has to post mandatory blind before the hand even begins.
For example, in a $1/$2 cash game, a player sitting in the small blind must post $1 before even getting his cards. On top of that, the player in this position will be the first to act in all postflop betting rounds.
This puts the small blind in a very unfavorable situation. Since the player will have to act without seeing what anyone else does before him and must invest money upfront, it is the worst position at the table.
To distinguish differences in cash games and tournaments, it is worth mentioning that both small blind and big blind is fixed in cash games and gradually change as levels progress in MTTs. For example, if you are playing in the previously mentioned $1/$2 cash game, the small blind will always be $1, and the big blind will be fixed at $2. On the other hand, tournaments will have a structure, and blinds will be increasing at fixed time intervals.