An anagram is a group of letters which when re-organised spell a word. Before being re-organised the letters can be just a jumble or they can spell another word or words.
A very simple example would be emlu which re-organised spells mule. They range from simple to very complex and often appear in crosswords. Another example maybe as a crossword clue would be something like “Can is upset with rival for fairground”. (8 letters). The words upset or disturbed or meaning any kind of turmoil usually suggests an anagram. In this case the word can with the word rival re-organised spells the word carnival which is the answer to the clue.