Web aplication testing in real life

I am working on a bigger platform based on php framework.

I was asked to think about testing it, at the beginning these should be automated tests.

I am reading about the testing and it is really confusing me. People keep writing and talking about "testing", they are only "testing" but I cannot get any specific information about what are tests actually about.

Most of the articles are directing me on Selenium but what does it actually do? I can record there how I'm clicking on my registration form and loop it for no reason?

This question might sound stupid but what is actually testing about? What can I test on a huge platform like the one I am working on right now?