Truncation is the shortening of a word to its root, e.g.
childhood - child*
This will find child, children, childhood etc
mammals - mammal*
This will find mammal, mammals, mammalian etc.
The root of a word can be searched often using an asterisk (*) or a question mark (?) to denote all possible endings.
Truncation should be used with care, as it can yield unexpected results, especially if the root is very short. For example: PIG* could very well find: