If you want to find word variants or both singular and plural, you can replace the end of a search word with a character. This is called truncation. The characters you use for this are called wildcard or joker and are usually an asterisk (*) or a question mark (?).
The effect of trunking depends on where in the word you are trunking.
In the example, you will find far fewer publications with childbirth* than with child*, but they are more specific. Because if your topic is birth, you probably won't be interested in articles about all children.
The character you use to truncate may vary from database to database. Sometimes the number of characters replaced with one character also differs. More information about this can always be found in the help of the relevant database.
Trunking within a word is called masking.