I just answered a similar question to this in another thread, so I'll copy/paste it here with some more info about "web3" specifically:

TL;DR: It's just a complicated (and energy intensive!) way of keeping track of things.

Web3, Bitcoin, Ethereum, and any "coin" you hear about on the web uses something called the "blockchain", which is just a MASSIVE file living on hundreds (thousands?) of computers around the world. This file is like a bank ledger: a record of things that happened. Think of it like a text file:

Bob gave Sarah €1.50
Sarah gave Alex €2.00
...

Now imagine that it's hundreds of millions of lines long, and every time anyone in the world gives anyone money, that list gets a little bit longer.

"But how do you make sure that people don't start tinkering with the ledger?" you might say? "I could say "Alex gives me €1000000". Well the files are kept in sync by this protocol where all participating computers do complex math to prove that they haven't edited anything. Everyone else does the same math, and so everyone's results should be the same. If your math is different, you're ignored. This is why transactions can take as much as a few hours or even days to go through and gobble a shittone of electricity. Awesome.

This is basically where Bitcoin came from.

After that was a thing, a bunch of other nerds got together and built Ethereum (same tech, different computers doing the work, so it's a different ledger), which uses the same technology. Ethereum however introduced this thing called "smart contracts" though, which are tiny programs that are baked into the chain (ie. they codified into this Great Big File That Everyone Has so they can't be changed). Smart contracts are simple instructions:

If Bob gives Sarah €1.50, Sarah then owns this: 1234567890

That 1234567890 relates to something in the real world, most famously a URL, which is where you get those NFTs that everyone was crazy about for a few months. Bob can give Sarah $1,000,000 and this would enshrine that Sarah owns https://somwhere.ca/picture-of-cat.jpg and then she can go around and say "I 'own' this picture". Then one day that website takes the picture down and Sarah realises that she paid a million dollars for a record in a text file.

Each transaction on a blockchain (Ethereum, Bitcoin, whatever) costs money, which you pay in that chain's currency, and that's where this all starts to make sense. If I can get you to buy my "magic internet money", then I'm selling you that currency for actual money.

Web3 is another abstraction on top of this, but most of it is nonsense. If you can use a contract to allocate ownership of something to someone, why not bake that into your website? The claim is that you can "take back the web" from centralised giants like Google & Facebook by "putting your data on the blockchain" and then you can choose to change who owns that data rather than these big companies.

It's a noble idea but tooooootal bullshit. First of all, the web is already open. I host my own website on a Raspberry Pi from my house. The idea that it's technology centralising the web and not capitalism is laughable.

Secondly, by design, everything on the blockchain is open. You get around this with encryption, but if I grant Facebook the rights to my data, they have it. If I change my mind tomorrow, then they still have it. They won't get any new data I append to my records, but I'm not "in control". It's actually much worse. The web3 nerds want us to store everything on the blockchain: browsing habits, mortgages, medical records, and all it takes is one unpatched update, your laptop gets hacked and now I own your house. Fuck That.

You probably haven't seen the definitive take-down of blockchain technology yet. It's long but solidly the best piece of criticism of the tech I've seen. Do watch it. It's far more authoritative than I can ever be.

Full disclosure: I bought into Bitcoin way back when it was cheap 'cause I was fascinated by the techology. Then I learnt how it actually worked, and how it absolutely cannot scale to be anything useful, so I just held onto the coins I had. I cashed out to the tune of about €40k. I absolutely would not recommend "investing" this this stuff. The domain is ripe with scammers and the project has no legs. It's long past the point of experimentation and is now at the stage of trying to drag more suckers in. Don't be that sucker.