Stances allow you to have combat and standard animations for holdables, the player can toggle between these states using CAP LOCK if the weapon has stances enabled. You can set a holdables stance animations in its holdable blueprint like for example the BP_M4A4 in the class defaults search for animations, there you can set the animations for third person and first person upper and lower body as well as the animations for each stance.
To enable stance switching for holdables search for Stance in there Class Defaults in the holdable blueprint here you can find the Allow Attacking in Standard Stance, which means if the weapon can attack, you will be able to attack while in standard stance.
Next is the Allow Stance Change which allows the player to switch stance when holding the holdable by pressing CAP LOCK.
And last is the default stance this is the stance the holdable will use when the player equips it.
There are additional settings for stances in the BP_PlayerInventory Class Defaults, search for stance to find them.
These allow you to set the default stance when no item is held and enable or disable Allow Stance Change when no holdable is held.
Stance animations for when no weapon is held are set in the character animation blueprints. You can read the Changing Character Animations guide for more information on that.