Heal block

Can someone please tell me what heal block is and how to use it please. Thanks

Heal block is an ability to stop the enemy from healing itself essentially. When you successfully apply heal block it shows up as a Red Square with a cross in it on the enemy and will also have red text saying 'Heal Block!'. Only a few select characters have it, some examples are -
the original Holy Smokes Eddie that you start the game with, his fury skill inflicts heal block on all enemies.
Grim Reaper Eddie has a good chance to heal block with his 'Clave" skill.
Rock Golem has a power skill that heal blocks all enemies.

Hope this helps :)