It is a (not frequently used) acronym for Input, Tools and Techniques, and Output. The PMBOK Guide defines the project management processess in terms of what goes in, what comes out, and how the inputs are transformed into outputs.
There are often a few questions on the PMP exam about these and candidates often ask if they need to memorize all these facts. Obviously it will help raise your score if you do, however a lot of questions can be answered with some common sense and general project management knowledge. So knowing that the project charter is an output of the initiation phase is the type of thing you will be tested on.