i assume you mean "heater core". this is a really small radiator usually located behind the dashboard of your car. a fan blows across it when you turn the heat on. this radiator can leak eventually, just like the regular radiator is prone to doing with age.
the difference is that to fix the heater core you have to tear the dash out of your car. i would just take it to a mechanic to fix, or buy the bypass kit at any auto parts store and cap it off (you won't have heat if you bypass it)
hope this helps!