Welcome to the forum noname1234. Hope it's okay to call you .
In a nutshell, interval work tears down / destroys muscle tissue. When the muscle is allowed to rebuild during the recovery stage it comes back stronger and more resistant to breakdown. That is how you get stronger. It's also the reason that recovery is critical to benefiting from intervals.
If you use the quick search function at the top of the screen you will come up with enough information to keep you in reading material for a quite awhile.
Hope this helps.
2003 Iceman Challenge - 2:34:55 - 897 / 2,000*
2002 Iceman Challenge - 2:39:23 - 1093 / 2,186
2000 Iceman Challenge - 2:49:18 - 1516 / 2,153