is a typical assignment for a pattern. Notice the presence of on the left-hand side, but not the right-hand side.
An assignment of the form sets up a transformation rule associated with the symbol f.
Different rules associated with a particular symbol are usually placed in the order that you give them. If a new rule that you give is determined to be more specific than existing rules, it is, however, placed before them. When the rules are used, they are tested in order. »
New assignments with identical lhs overwrite old ones. »
You can see all the assignments associated with a symbol f using ?f or Definition[f].
If you make assignments for functions that have attributes like Flat and Orderless, you must make sure to set these attributes before you make assignments for the functions.