SPSS ships with a manual called "Command Syntax Reference" (or CSR). Whenever you don't know or understand something, this should usually be the first place to look.
SPSS Command Syntax Reference
"If Everything Else Fails, RTFM"
How to Use the CSR?
All commands are listed alphabetically. If you know which command you need, it may be fastest to navigate to it by means of the bookmarks. One could either start reading at its start or expand the bookmark to find a more relevant starting point. Otherwise, one could navigate to the first page of the document and use CRTL + f to find some keyword or expression. Note that some very helpful information is also found under Universals, especially under Transformation Expressions.