What is a common 'map frame'?

I'm going through pcl's tutorial on How to incrementally register pairs of clouds and I came across the following sentence in the introduction:

Your data set should consist of clouds that have been roughly pre-aligned in a common frame (e.g. in a robot’s odometry or map frame) and overlap with one another.

What do they mean by a common map frame?

I'm going to be using an input from a depth cam/lidar which will provide me with a consecutive depth frames at 30fps, is each pair of following depth frame already in a common map frame?

If not, what is the process of combining them into such?