A catalyst is a substance that speeds up the rate of a reaction without altering the products of the reaction. A catalyst increases the rate, but the same products are made as without it.
A catalyst is not used up. At the end of the reaction it is chemically unchanged and its mass is the same as at the start. If 2.0 g of a catalyst is added to a reaction, 2.0 g can be recovered afterwards by filtering and drying it. Because it is not used up, it does not appear in the equation for the reaction, and a small amount can be used again and again.
A common example is the breakdown of hydrogen peroxide into water and oxygen: 2H2O2 → 2H2O + O2. This happens slowly on its own. Adding a small amount of manganese(IV) oxide makes oxygen bubble off much faster. The manganese(IV) oxide is a black solid that can be filtered off at the end with the same mass as before.