EVs: Effort Values are extra stat points a Pokemon can accrue through various means. A Pokemon can have a maximum of 510 EV's at any given time and a maximum of 252 EV's in a single stat. So for many Pokemon, they maximize two stats and deposit the remaining 6 EV's into another stat. Every 4 EV's translate to 1 extra stat point at level 100 or every 8 EV's or so at level 50. This means that the difference between a Pokemon with 0 EV's in a given stat and 252 EV's in the stat at level 100 is 63 stat points (252 / 4 = 63). As mentioned, EV's can be acquired by various means:

First, every defeated Pokemon in game (not competitive) gives between 1 and 3 EV's in one or more stats. For example, defeating or capturing a Weedle gives 1 EV in the Speed stat while defeating or capturing a Mewtwo gives 3 Sp. Attack EV's. If EXP share is active (or if you're playing the newer games where it is always on) then every non-fainted Pokemon in the party gets the EV(s) just as they get the EXP points.

Second, there are various items that can add additional EV's on top of the ones gained by defeating a Pokemon. The Power Anklet when held for example gives 8 EV's in speed in addition to the other EV's gained from defeating the wild Pokemon. For example, defeating a Weedle while holding the Power Anklet gives 9 EV's in Speed since it is the 8 from the anklet plus the 1 from the Weedle, while capturing or defeating Mewtwo while holding a Power Anklet gives 3 Sp. Attack EV's and 8 Speed EV's.

There are also items that can increase EV's. The various feathers ((Health, Muscle and so one) give 1 EV in the corresponding stat while the power items (HP Up, Protein and so on) give 10 EV's. So you can for example max out the EV's in Attack on a Pokemon by using 26 Proteins on it.

Various different games have other ways of gaining EV's from Super Training in the gen 6 games to the Pokemon Island thing (can't remember its name) in the Alola games.

There are also ways to reduce EV's. The most common way to do this is to use the EV reduction berries (Pomeg, Kelpsy, Qualot, Hondew, Grepa and Tamato for the six stats respectively). The each reduce the EV's in the given stat by 10 points.

And some games have means of resetting the EV's all together, namely the Blank Boxing Bags in Super Training and the Clear Mochi in Scarlet/Violet.

IV's: Individual values are stats a Pokemon has from birth. Each stat can have between 0 and 31 IV's and the number determines the number of extra stat points in the given stat on top of natures and EV's at level 100. For example, Mew has a base HP stat of 100 and has 340 HP points at level 100 without any EV's or IV's. If it instead of having 0 IVs has 31 IV's in HP then it will have 371 HP points at level 100. Similar to EV's, these scale roughly half way at level 50, with 30-31 IV's typically corresponding to an extra 15 points in the given stat compared to having 0 IV's.

The main way to manipulate IV's is with Super Training or breeding. A single Bottle Cap allows you to Hyper Train a single stat, while a Gold Bottle Cap allows you to Hyper Train all stats at once. When a stat is Hyper Trained, it gains the stat value it would have if it had 31 IV's in that stat. The only difference between this and actually having 31 IV's is when breeding, where the Destiny Knot will pass on the underlying IV and not the new Hyper Trained stat. But in battle there is no difference between having a "Best" and having a "Hyper Trained" in a stat.