|There is a device on many guns called a safety.|
This is the safety from my Black Aces shotgun on "safe" or white, the trigger can't be pressed, it will be locked in place.
The safety is on "red" or "fire", so the trigger will work if pressed.
Red means fire and white means safe. The pointer is pointing to white which means the gun is safe, which means you can't press the trigger. If the lever is down or pointing to red then the trigger will work if it's pressed. The expression "Red means dead" can also be used. When you're not firing your gun you should make the gun "safe" by putting the safety on white. When you're ready to fire, turn off the safety. You will often be ready to fire your gun but it won't work because the safety is on. Get into the habit of turning off the safety when ready to fire and turn it on after finished.
Some guns don't have a safety mechanism.
Safeties can be in different positions in different guns so learn to use them, most are near the trigger and will have a red/white indicator.
If you can't pull the trigger it might be due to the safety being engaged.
Don't rely on a safety as it locks the trigger but doesn't necessarily prevent a gun from being fired if it's dropped.