Answer:
Qibla means the direction to which the Muslims pray - facing the Kaaba of Mecca. Now, you do know that there is nothing on the wall which they face while praying, so, they need something like a pointer to reassure them that they are praying in the right direction, for which there is a small semi circular recess made in the middle of the wall that they must face which is like a pointer saying "Mecca, this way" and that's called the mihrab. And, no, the wall is not filled with mihrabs. It's usually just one in the middle.