Initiative is one of the Attributes in Divinity Original Sin 2. These statistics are determined by your character's base statistics. They can be affected by skills, stances, statuses, gear, abilities, and talents.
In Divinity Original Sin 2, Initiative is used to determine who gets to move first in battle. Note that D:OS2 is using a "round robin" system where no matter your Initiative, turns will always be taken in an alternating fashion between allies and enemies.
Even if your 4-man party of allies has an initiative of 44, 43, 42 and 41 respectively, meeting a 4-man group of enemies with an initiative of 4, 3, 2, 1 respectively, the order of taken actions will be distributed evenly.Using the above example of allies (A) and enemies (E), the order of taken actions would be: A44, E4, A43, E3, A42, E2, A41, E1.
What this means is that with initiative, you are only really deciding the order of movement and actions taken within a given sub-group, with little-to-no effect on how combat will play out. For all intents and purposes, initiative is largely useless, and should never be prioritized.