Parallelism occurs when successive clauses or sentences are similarly structured. This similarity makes it easier for the reader / listener to concentrate on the message.

Example:

The mediocre teacher tells, The good teacher explains. The superior teacher demonstrates. The great teacher inspires. (William A. Ward)

Note: When writing, parallelism is a useful device for instructions. Due to the parallel structure, the reader can concentrate on the message and will immediately know what to do.