You have a finished product but you have time left before you release it to the public.
So you contact a segment of the public who are interested in your software, and who are also willing to provide input to you on how the software works, free of cost to you.
You just release a pre-retail version of the product to these people who evaluate it and let you know of the shortcomings.
This is called Beta Test/Beta Testing.
Then you fix the shortcomings and release it to the general public.
If you release the software to people of your own company to test it, it is called Alpha Test/Alpha Testing.