Runes are required to level up your stats. You can get Runes from defeating enemies or bosses. Each stat you level up increases your level by 1.

Vigor:

Increases maximum HP and fire resistance.

Increases Immunity, making you more resistant to Poison and Scarlet Rot.

Mind:

Increases maximum FP.

Increases Focus resistance, making you more resistant to Sleep and Madness.

Endurance:

Increases maximum stamina. Your stamina will not deplete in the open world unless aggroed by enemies.

Increases physical defense.

Increases Robustness, which is your resistance to Hemorrhage and Frostbite.

Increases your maximum equip load.

Strength:

Increases damage of strength-scaling weapons.

Required to equip heavy, Strength-based weapons.

Dexterity:

Increases damage of dexterity-scaling weapons.

Required to equip advanced, Dexterity-based weapons.

Reduces casting time of spells

Decreases fall damage

Makes it harder to get thrown off your mount.

Intelligence:

Boosts damage of intelligence-scaling sorceries and weapons.

Required to perform more advanced sorceries.

Improves resistance to magic attacks.

Faith:

Boosts power of faith-scaling incantations and weapons.

Required to perform more advanced incantations.

Arcane:

Boosts discovery, which increases rare item drops.

Raises your Death resistance.

Increases blood loss buildup for weapons that both have Arcane-scaling and blood loss buildup.

Which Stats should I level up? - Elden Ring Raiders