Explain the term- Scan Insertion

Scan Insertion is completed by a tool and results in all (or most) of your design's flip-flops to be changed by special "Scan Flip-flops". Scan flops have additional inputs/outputs that allow them to be configured into a "chain" (e.g. a big shift register) when the chip is put into a test mode.

The Scan flip-flops are linked up into a chain (perhaps multiple chains)

The ATPG tool, which knows about the scan chain you've formed, generates a series of test vectors.

The ATPG test vectors contain both "Stimulus" and "Expected" bit patterns. These bit vectors are shifted into the chip on the scan chains, and the chips reaction to the stimulus is shifted back out again.