A PSI (Program Structure Interface) file represents a hierarchy of PSI elements (so-called PSI trees). A single PSI file (itself being a PSI element) may expose several PSI trees in specific programming languages (see File View Providers). A PSI element, in its turn, can have child PSI elements.
PSI elements and operations at the level of individual PSI elements are used to explore the source code's internal structure as it is interpreted by the IntelliJ Platform. For example, you can use PSI elements to perform code analysis, such as code inspections or intention actions.
PsiElement class is the common base class for PSI elements.
How do I get a PSI element?
What can I do with PSI elements?
See PSI Cookbook.