What is a semiautomatic weapon?

A semiautomatic weapon is one in which one bullet is fired each time the trigger is pulled, until the magazine is out of bullets.

These types of weapons are often called "auto loaders" because part of their functioning is to load a fresh round into the chamber for the next shot.

A typical semiautomatic firearm functions in the following manner:
1- Loaded magazine is inserted into the weapon
2- A round is chambered (cocking, racking the slide, etc...)
3- The trigger is pulled, a hammer hit a firing pin (or striker) which fires a bullet
4- Either the recoil energy, or gas pressure is used to automatically extract the spent brass
5- This spent brass is ejected from the weapon
6- A fresh round is stripped from the magazine and chambered
7- The hammer (striker) is also reset automatically

SO the shooter only needs to pull the trigger again to shoot another bullet. In an automatic firearm, MORE than one bullet can be fired with each pull of the trigger. The same functions occur, just that a mechanism called the "sear" doesn't catch the hammer and prevent another shot without releasing and re-pulling the trigger.