Usually measured in millimeters, the bottom bracket rise or bb rise is the distance between the center of the bb shell and the axel to axel line.

For example on this XTP frame kit:

although the rise on this frame isn't always 100% accurate due to the horizontal dropouts. geometry of forks can also affect the rise of the bb.

A easy way to find out your bike bb, is to tie a peice of string on both the axels and pull it as tight as it will go. Then measure up from the string to the centre of the bb.