All characters have a basic capacity to performed actions, due to their ability scores. If you are untrained in a skill, armor type or weapon group, you use your base ability modifier as your check modifier.
Certain weapons, armors, skill actions, and other actions are listed as trained only, or even expert only. If an action is listed as trained only or expert only, you grant advantage while attempting the action if you do not have training. If a weapon or armor is listed as trained only or expert only, you grant advantage while wielding that weapon or wearing that armor.