He was called a documentarian, an experimentalist, but I preferred to call him an "exploratory" filmmaker.
One of my favourite Mekas works is Notes on the Circus (12 minutes, 1966). It starts. "I get it. He shot film at a circus." But after a few minutes it becomes hypnotic.
This film school graduate imagines that Jonas Mekas would have been a fascinating lecturer and guest instructor.
From The Guardian: Jonas Mekas: how a Lithuanian refugee redefined American cinema