Often in a poker game, a player will make a big river raise representing the nuts or strongest hand in poker. This will even come from a very good player that 99 percent of the time does this while possessing decent poker hand and is only bluffing the other 1 percent. A hero call is when a player realizes when that 1% is happening and makes this very difficult call.
To be a little more precise, a Hero Call is when a poker player makes a risky marginal call solely based on a read or gut feel.
The key problem with hero calling is that there is an incredibly thin line between making the odd, balanced big call, and of turning into a massive calling station. Most of the time in poker, big bets and lots of action indicate big hands, and starting to call big river bets in Texas Holdem with marginal hands can become an extremely dangerous game.
You need to have a very good specific reason in your head to make these hero calls, and just doing them on an impulse is usually a recipe for disaster. Mostly you should be looking to make these calls either:
a) To slow down a very aggressive player; at some point you will have to make a stand against these players, and there are some situations where calling with marginal hands are actually quite good spots to achieve this; or
b) Because of a specific read on a player that makes you extremely certain that they are up to something. Be it a timing tell online, or a physical tell live, you usually need some kind of reliable evidence to make very big calls in the face of lots of action.
Think very carefully before making a hero call otherwise it might end your game a bit too early.