Overrun means being overwhelmed or overcrowded with too much of something.
Imagine your kitchen counter is cluttered with dirty dishes and appliances, so much so that you can't move around without bumping into something. That's what it feels like to be overrun. It's when a place, system, or even person is completely taken over by too many things to handle. This can happen in traffic, too - if roads are clogged with cars and it takes forever to get through the intersection, you're likely overrun.
How common is "overrun"?
Word overrun is considered rare in modern English. It has a balanced usage among all categories: speech, fiction, newspapers and academic texts.
Definitions
verb
(transitive) To defeat an enemy and invade in great numbers, seizing the enemy positions conclusively.
(transitive) To infest, swarm over, flow over.
Example: The vine overran its trellis; the field is overrun with weeds.
(transitive) To run past; to run beyond.
Example: The athlete overran the finish line and kept going.
(intransitive) To continue for too long.
Example: The performance overran by ten minutes, which caused some people to miss their bus home.
(printing) To carry (some type, a line or column, etc.) backward or forward into an adjacent line or page.
(transitive) To go beyond; to extend in part beyond.
Example: In machinery, a sliding piece is said to overrun its bearing when its forward end goes beyond it.
(transitive) To abuse or oppress, as if by treading upon.