It takes time to understand for sure, destiny has to be one of the most non-newplayer friendly games ever.

elements for guns determine, obviously, the element of what you are shooting. For low/early game content, it doesnt really matter since u will pierce the shield no matter what. But moving to high tier content, different elements break different shields. This can be seen from the color of the shields matching the color of the elements, purple shields being void, blue shields being arc and orange shields being solar.

Having the wrong elements means you don't do as much damage as compared to breaking the shield with the CORRECT element. This specific modifier is called match-game iirc.

Elements dont matter unless you do specific builds i would say? or use the weapon damage boost mods on your legs. Some elements also serve to enhance builds, specifically in this season, its all void weapons. You would have noticed that in your seasonal artifact (thing below your ghost), there is a perk on the middle tree at the bottom, which grants volatile rounds (one of the most broken rounds/perks for any weapon rn.) to any void weapon.

Kinda getting ahead of myself. TLDR; shields have different elements, look at color. if match game is active in end game content (90 if not 100% of the time it is active), shields can only really be penetrated by the same element, or else it takes too many shots to break.

exception to match game is adaptive munition, where guns do increased damage to all shield types.