A macro is a recording of typed commands that can be executed later. Macros are stored in registers; recording a macro can be done with q, and you can run them with @.
The documentation for this feature can be found at :help complex-repeat.
N.B. In most of the Vim documentation, "macro" is used as a synonym for "mapping". However, the Vim community mostly uses the term to refer to Vim's "Complex repeats" feature.
