Why do we need apache ? How does php and apache work together ?

I have just started learning PHP and one thing i am still confused about is what exactly is apache? if its a web server what does it mean that more than 60% of the web is hosted on it, and why do we need to have apache packages like xampp to test php while we can use sites with php online(web browser) without having apache installed? P.s: sorry if this is dumb question but it has frustrated me. This is also my first question on stack overflow so please excuse any mistakes i have made,any help will be greatly appreciated!