What is Decentralized Finance (DeFi)?

Help a newbie out: What is Decentralized Finance (DeFi)?

So, I've hit a massive, painfully expensive brick wall. I really need someone here to explain something terribly basic for me: exactly what is Decentralized Finance (DeFi)?

I'm completely stumped.

Last Tuesday, I tried skipping the research phase to swap a tiny chunk of Ethereum for some obscure altcoin on Uniswap—mostly because a coworker swore it was a guaranteed moonshot—and I miraculously paid seventy-five bucks in network gas fees for a transaction that just flat-out failed.

Poof. Gone.

It stung. Badly.

That ridiculous rookie mistake made me realize I skipped step one entirely. I lack the absolute foundational knowledge to survive out here. If somebody asked me right now, "Hey, what is Decentralized Finance (DeFi)?" I'd just mumble something pathetic about magic internet money and rogue algorithms.

Is it literally just a giant, unregulated casino running on open-source computer scripts? Or is there an actual, tangible replacement for my boring local bank hidden underneath all this chaos?

Here is exactly where my brain breaks down:

• Smart Contracts: Are these things genuinely firing off without a single human hitting an "approve" button somewhere?
• Liquidity Pools: How is generating a twenty percent return mathematically sustainable without a central bank? Where does that cash actually originate?

My Current Mental Model

I desperately need practical, usable advice before I attempt another swap. Can a veteran please break this down? When you strip away the Twitter hype, what is Decentralized Finance (DeFi) at its functional core? Any gritty, battle-tested resources you can share would save me from burning another seventy-five dollars into the digital void.

Thanks in advance!