Trim() is a function to strip whitespace from either side of a string variable, with "whitespace" meaning spaces, new lines, and tabs. That is, if you have the string " This is a test " and pass it to trim() as its first parameter, it will return the string "This is a test" - the same thing, but with the spaces trimmed off the end.
You can pass an optional second parameter to trim() if you want, which should be a string specifying the characters you want it to trim(). For example, if we were to pass to trim the second parameter " tes" (that starts with a space), it would output "This is a" - the test would be trimmed, as well as the spaces. As you can see, trim() is again case sensitive - the T in "This" is left untouched.
Trim() has two minor variant functions, ltrim() and rtrim(), which do the same thing but only trim from the left and right respectively.