Beta is the second letter of the Greek alaphabet. It also refers to a preliminary model of software or hardware that's not a finished project.
There are two related meanings of beta: the first is the second letter of the Greek alphabet. The second refers to things that are secondary in a different way: the beta version of software or hardware is a version that isn't quite ready for prime time. The beta model is a testing version. Often a company will let some people use the beta version to see if there are problems which can be fixed before the final version comes out.