A hex is a curse that is cast by someone based on their actions or a comment they made to which you may respond "Hex!!!!" pointing out what they've done. But it can also be used by someone to sympathise with someone else over something that has happened.
There are additions to Hex that are used to further detail the nature of the Hex:
Cock Block Hex - When you get cock blocked
Self Hex - When you cause your own Hex
Reverse Psychology Hex - Said when someone is trying to use reverse Psychology on you
Person 1 "I'm not being funny but no-one got laid in Cancun...."
Person 2 "HEX!!!! now we're never gonna get laid
Person 1 "My bitch won't let me go to cancun..."
Person 2 "HEX!!!!"