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.
