Actually Beta is the 2nd stage for a program.
Alpha, Beta RC also known as Release Canidate.
Depending on what the program is and who it is from. Some companies only do Betas and skip the Alpha and RC stages while others do Alpha and Beta before release.
Mozilla is a Alpha, Beta company. Microsoft does all 3. But their Alpha are called Milestone Builds.
But to specifically answer the question a Beta is a program that is still in testing that is not qualified for everyday use. There are still bugs, and glitches in the programming tht can make it not work on your system, cause havoc on your system, or not work as expected.
If you are not familiar with Beta testing, no offense but asking what a Beta is qualifies as just that, i would steer clear of anything with the Beta Label. It might not work or cause corruption on your system. Last thing you want.