A catheter is just a fancy word meaning a medical "tube" of some kind.
Some are for general drainage (like the ones inserted into the bladder, called Foley Catheters) while others are for more specific highly specialized diagnostic or treatment functions (like cardiac catheters, used to balloon open clogged heart arteries).
In the case mentioned in your citation, a tiny sterile tube, guided by a metal wire, is snaked into the coronary arteries to the point of the blockage causing the heart attack. Once it gets there, it will expand open, spreading open the clot blocking the artery and reestablishing vital blood flow to the cardiac muscle. Often a "stent" (small metal wire mesh) is then placed to permanently hold open or prevent the clot from collapsing again.
The tubular catheter is what allows the tiny medical devices to be delivered directly to the spot of the blockage, where they can do their work.