Here's a breakdown:

* Protostar: A protostar is a clump of gas and dust that is beginning to collapse under its own gravity. It's still not hot enough to start nuclear fusion, which is the process that powers stars.

* Nuclear Fusion: This is the process where hydrogen atoms fuse together to form helium, releasing tremendous energy. This is what makes stars shine.

So, a protostar is like a baby star, still forming and gathering mass before it can truly ignite and become a star.