What is the SPV Mining Exploit? Can anyone explain this simply? I am not finding enough explanation exactly on this type of attack. If possible, add some additional reading to your answer so that I can understand more.
"SPV mining" refers to a bad practice where a miner starts hashing a new block on top of an unverified parent block. If the parent block turns out to be invalid due to a double spend or newly activated soft fork, the new block will also be invalid.

The term SPV in this case is used because the miner only verifies the headers of the incoming block and then immediately starts mining the next block. 

This has happened on main net before: when BIP66 activated in 2015, a short chain of blocks was mined on top of an invalid block that did not enforce the new rules. 

This question may be a duplicate:

What is SPV mining, and how did it (inadvertently) cause the fork after BIP66 was activated?