A catalyst is a substance that increases the rate of a chemical reaction without undergoing any permanent change itself. Catalysts work by lowering the activation energy [minimum energy required to start a chemical reaction] required for a reaction to occur. For example, enzymes [proteins that act as catalysts in biological reactions] in our body make it easier for other chemicals in our body to produce the reactions that break down food into energy.
Very Simple Definition:
A catalyst is something that makes chemical reactions [changes that chemicals cause to other chemicals] happen faster. It is like a helper that makes things easier and faster without participating itself. For example, when we add yeast to bread dough, it helps the dough to rise faster and become fluffy. But the yeast remains yeast.