OK, so the first thing to understand is that IVs and Evs are two separate things. IVs are predetermined values which can directly influence a Pokemon's stats. Evs, however, are not locked, and can be changed by the player. Each Pokemon can have a max of 31 IVs per stat, meaning that it is possible to have a Pokemon (say Bulbasaur) with 31 IVs in each stat. However, it is also possible to have a Bulbasaur with 0IVs in each stat. And you could have a Bulbasaur with any number in between per stat, meaning that I could have a Bulbasaur with 31 IVs in Special Attack, but only 5 in Special Defence. IVs have a direct effect on the Pokemon's stats, for instance if I was using a Level 5 Bulbasaur vs another Level 5 Bulbasaur, and they both had a neutral nature (say Bashful) and mine had 31 IVs in Speed while my opponents' had 23 IVs in speed, mine would go first round as it has more IVs in Speed. IVs also affect the type of a Pokemon's Hidden Power Move, to where it can be any type in the game. A select combination of IVs gives a particular Hidden Power, for instance a 6IV Pokemon (31 IVs in every stat) has a Hidden Power Dark.
Now for EVs. EVs are short for effort values, and they can also influence a Pokemon's stats. Each Pokemon can have a max of 510 EVs altogether, and these can be spread any way the player wishes. Each stat can only hold a maximum of 252 Evs, meaning I could have a Bulbasaur with its Evs assigned as follows:
252Evs in Special Attack
252Evs in Speed
4Evs in Defence
If you were wondering where the last 2 Evs are, they are left out as Evs are assigned in groups of 4, as 252/4 is 63 and 4/4 is 1. You can also spread your Evs any way you wish, meaning I could remove 52 Evs from Bulbasaur's speed and dump them in Defence, meaning Bulba now has 200 Evs in Speed and 58Evs in Defence. EVs and IVs can combine, meaning that a Bulbasaur with 31 IVs in Speed and 252Evs in Speed (plus a speed boosting nature such as Jolly) would be the fastest Bulbasaur it is possible to have at its level. Evs can be changed by using Poke Pelago in USUM and SM, or by using the Ev drinks (Protein, Zinc, Carlos, Iron, HP Up, and Calcium), which raise specific stats' Evs by 10 points.
As to whether Evs are worth it, yes. Definitely yes. they give you so much control over your Pokemon. They let you max out their stats, and make the best competetive team. Ivs help too, enabling you to further customise your Pokemon. Also use natures, which raise one stat by 10% at level 100, and lower another by 10% at level 100. Don't use Special Attack boosting natures on Physical Attackers such as Mega Mawile, and don't use Attack boosting natures on Special Attackers such as Mega Charizard Y, then you should be fine. Also, try breeding for 31 Ivs in a particular stat, as it is a huge help in competetive play. That should be it, if I've missed anything can somebody let me know please!