On a bottom bracket, what does the numeric expression e.g. 1.37x24 mean? On a bottom bracket I can read 1.37x24 - what do 1.37 and 24 stand for?
It describes the thread of the bottom bracket.

1.37 stands for the thread diameter and is measured in inches. 24 stands for the pitch size (the distance of the thread in one turn). It's measured in TPI, threads per inch.

If the values differ, a bottom bracket won't fit to the frame.