What is a share?
A share is a part of a company. If I start a company, I can sell small parts of it to people. A share is one slice of a pizza. You can buy as many slices as are available.
We'll think of a share just as a thing that we can buy and then sell to others.
Company A makes bicycle parts.
Company B is an online shopping site.
Company C is a software company.
If these companies are on the share market, we can buy and sell their shares in the same way we buy and sell pizza slices.