How can I "exploit" in Elder Scrolls Online and what does it do?

Whenever someone else is attacking an enemy, I sometimes get a message on the screen that says

Hold (Right Mouse Button Icon) to Exploit

What does that mean exactly and what does it do?

What are the effects of "exploiting" in this case?

"Exploiting" is a game mechanic that will essentially knock-down/stun an opponent.

To performan a successful exploit, you'll first have to get your enemy into off-balance (e.g. by blocking a heavy attack or disrupting a cast). Once that happens, you're able to perform a heavy attack (doesn't have to be fully charged) to exploit their weakness, resulting in the mentioned stun/knockdown.