The word “creel” refers to the wicker basket used to carry fish, and “creel limits” is a general term referring to the number of fish you're permitted catch. Laws govern the numbers of fish you can keep each day as well as the total number you can have in your possession for the entire fishing trip. These laws are based on the numbers of fish in certain bodies of waters and how fast they can reproduce and rebuild populations. Limits are set to ensure the fish have a sustainable population for future fishing.
Daily limits control the number of fish you can catch and keep in one day. The numbers vary widely depending on how prolific the fish are in a particular body of water. Some species like crappie and whiting have very high limits; others like bass and redfish are much more restricted. The biologists at the Department of Natural Resources set these numbers, usually with some input from fishermen.
Creel limits are on a per-person basis, and each person fishing can keep that number. Some people get around the limits by taking young kids and other nonfishing family members with them and catching the limit for each person; but ethical fishermen and game wardens frown on getting around the law this way.
Some species of fish may be so plentiful that there is no limit on them. If an undesirable or non-native species of fish gets started in a body of water, to encourage elimination of the species, often no limits are set on them to encourage fishermen to keep all they catch. If a species of fish has a high limit, that tells you they should be easy to catch and you shouldn't worry about keeping as many as you want.
Laws governing fishing also control the number of fish you can have in your possession. Sometimes the possession and daily limits are the same but often the possession limit allows you to have more fish. This allows fishermen making weekend trips to keep a limit each day, up to a certain total for the whole trip. The limit is usually low enough, often two or three times the daily limit, so that fishermen can't keep huge numbers of fish and claim they've been fishing for a long time.
Possession limits also allow fishermen to take advantage of seasonal fishing patterns where one kind of fish may bite good for a few days then disappear for the rest of the year. The authorities try to strike a reasonable balance between allowing a fisherman to keep fish to eat and not harming the fish populations. Most fishermen don't try to keep large numbers of fish, and sport fishermen usually don't keep enough fish to be concerned with possession limits.