Mechanical activities are activities learners do which are not meaningful but which may be necessary in order to have the tools to use language. The move away from mechanical to communicative activity has been emphasized and prioritized for many years.
Learners thinking about the position of the tongue in the pronunciation of certain sounds, or learning how to write in Roman script are doing mechanical but necessary activities.
In the classroom
Drills, substitution exercises, and using prompts to produce structures are common mechanical activities.