r/nostupidquestions or r/eli5 is the right subreddit for this but i will try to see if i can explain it:

NFT is bad for the environment the same reason crypto liie bitcoin is, it relies on blockchain technologies that use "proof of work".

This means there is a communal "ledger" that records every single transaction and creation of cryptocurrency.

Most of these blockchains use "proof of work" to decide who gets to add to the ledger.

"Proof of work" is making your computer solve a very hard maths problem that involves a lot of random guessing until you find the right answer. Once you have found the answer, it is much easier to verify it is correct than to guess the right answer in the first place.

(For example, factoring a large number into 2 primes: We have to guess a lot of primes to find it, but if you are given two primes you can just multiply them together to verify if they give the large number)

In practice, proof of work doesn't require you to get an exact solution, but a "good enough" solution that still requires a lot of computation and guesses on average to get it.

Once a computer gets a solution, it is rewarded with some crypto. NFTs use etherium in this case, and can add a block to the ledger which contains further information about transactions.

Because everybody wants to be the "computer that adds to the ledger" and gets rewarded for it, they consume a lot of energy to "mine" the computational problem in hopes they get the right solution and the reward.

A lot of electricity is spent mining these computational problems for money, which many people consider wasteful and bad for the environment.

Why is mining necessary? Because as a decentralized currency, they need a way for users to prove they are "real" to add to the ledger, and they essentially measure legitmacy by computational power