Raw experience refers to the amount of experience you are pulling without any additional experience bonus. For example Premium Bonus or Experience Boost from the shop, or Prey Experience Bonus. So Raw experience is the definitive best way to know what spawn gives the best experience when you are looking for "pure" experience with no bonuses.
This is especially helpful with players trying to make guide videos on where the best "leveling" spawns are so you can gauge how much experience the spawn can give without having to "min max" by only hunting premium bonus with boost and experience prey.
As far as events or things "impacting" raw experience, most events don't directly increase raw experience since "raw" means the exact amount of experience the creature gives without any outside boosts. So if a creature gives 1,000 experience raw then the outside boosts are only adding to the experience for your specific character, not adding experience to the creature, which is why 1 person in the party can have premium bonus and boost and receive 4,000 experience from the creature and somebody else in the party could have no bonuses and only receive the 1,000 experience. The only events that would affect the raw experience would be ones that directly increase the amount of experience a certain creature gives, for example "Spring into life" event that increase experience for Dragon Hatchlings.
Other than events like that you could technically "increase" the amount of raw experience your pulling in a spawn during a "rapid respawn" event since you will more than like be killing double the amount of creatures in the same amount of time. So although the creatures aren't giving you more experience, you are technically killing more of them and faster which would mean you are going to pull more experience overall. For example, If i were to hunt a spawn at level 500 and pull 5kk raw, then go back to the spawn at level 800 I am more than likely going to be pulling more raw exp an hour than before since I am now stronger and faster and will be clearing the creatures quicker.
Hoped this explanation helped!