Mitigation is % value of how much damage you will mitigate when receiving a hit.
If you have mitigation 2% and receive hit for 1000, you will mitigate 20 dmg and receive a hit for 980 (we are ignoring armor or other resistances for now).
Mitigation comes from weapon/shields and its defense values and the stance (offensive/balanced/defensive).
What you can see in the gems or wheel of destiny is not a mitigation, but a mitigation modifier. lets say you get 15% mitigation modifier from your wheel/gems
it will make your mitigation from 2% to 2,3% (2%*1,15)
When receiving a hit for 1000 dmg with mitigation 2,3% you will mitigate 23 damage.
with mitigation 2% and mitigation modifier 24%:
receiving a hit for 1000 dmg with mitigation 2,48% you will mitigate 24,8 damage.
it is up to you to decide whether it is worth it or not.
Do not confuse mitigation and mitigation multiplier, they are two different parameters. Mitigation multiplier can be found in the wheel of destiny and your total mitigation(mitigation including the modifiers ) is visible in your character tab.