Something immobile can't move or be moved — it's locked or stuck in place.
If you know that mobile things move around, then you're halfway to understanding what immobile means: this refers to anything that can't move. A mountain is immobile; no one's moving that. Sometimes people who are very large — like offensive lineman — are described as immobile, because they are big and hard to move. When something is immobile, it's not going anywhere.