Idioms are groups of words that often defy grammatical rules and mean something other than what they appear to mean.

kick the bucket (someone has died)
under the table (pay someone secretly)
in the bag (finished, completed)

While informal expressions such as these are seldom used in college writing, they are part of a flexible, effective, all-purpose vocabulary.

TIP: The best way to learn idioms is to listen carefully and read extensively.

For more information on idioms, check your library's dictionaries.