What is an IT block or IT instruction?

Reading the tm4c1294ncpdt.pdf  I found a discussion about IT folding and IT blocks.  

What is an IT block?  

Here is the detail from the PDF:

Register 54: Auxiliary Control (ACTLR), offset 0x008

In some situations, the processor can start executing the first instruction in an IT block while it is still executing the IT instruction. This behavior is called IT folding, and improves performance, However, IT folding can cause jitter in looping. If a task must avoid jitter, set the DISFOLD bit before executing the task, to disable IT folding.

Thanks