Definition of Fixed Limit
When you play a poker variant that is "fixed limit" that means that the limit on raises is fixed at all stages of a hand. When you play no-limit hold'em, at any stage of the hand you can raise any amount you like. The only constraint is that you must have a big enough stack to make your raise. Basically, your stack constrains the size of your raises. When the variant is Fixed Limit, you will have to follow the rules of that variant to determine your maximum raises at each stage of the hand.
Fixed Limit vs No Limit
Since 2003, no-limit hold'em has been the most popular form of poker. Prior to that time, in Las Vegas casinos, Fixed Limit Hold'em was very popular. It is less exciting than No Limit Hold'em, but has less variance and fewer bluffs. You can play both Hold'em and Omaha or Stud in Fixed Limit. Generally, we will play at higher limits in Fixed Limit. For example, a NL25 player (blinds at 0.25/0.50) in No Limit Hold'em can play with 1/2 blinds in Fixed Limit Hold'em.