What’s a math concept that feels completely different once you truly understand it?

I’m curious about those topics that seem like a bunch of rules and formulas at first, but suddenly make much more sense once you understand the idea behind them.

For me, calculus started feeling much more intuitive once I stopped thinking of derivatives as just a formula and started thinking about them as describing how something is changing.

What math concept had that “ohhh, NOW I get it” moment for you?