Tldr: What are smart Contracts?

I feel like a lot of people know a lot of the fundamentals of Blockchain and Bitcoin technology, and even more so of practical use of Ethereum, Matic, stellar, etc. But they don't know the basics of smart Contracts! So here!

Smart Contracts make sure that when using pay to pay applications you can ensure that both people will be paid.

Say for example you have a fundraiser with smart Contracts. It's the intermediary that moves the funds from several backers to the company / idea that needs money.

Since Smart Contracts work on Blockchain it means once a smart Contract is made it cannot be undone. And smart Contracts must be validated by everyone in that block. Meaning that it's impossible to manipulate a smart Contract.

So what's the use of smart Contracts? It's very useful for payments that occur frequently, and need validation that one paid their fair share. Smart contracts main purpose is a validation, it can validate if someone has enough money, if someone did an action, if someone paid a loan, etc.

After a smart Contract is done the ledger is updated.

tldr for the tldr: A smart contract is useful to validate needed things, Like checking if someone paid a loan, or paid a monthly cost, like bills or something. and it acts as the anonymous intermediary for moving money from person to person since it works on the Blockchain...

Edit: user Kenarion in the comments gave more examples if you need further help! Thanks! Also i appreciate anyone who gives more real life examples, more coding examples, it's all really useful, I'm reading them all, i love them all!