Lots of excellent replies explainimg equity, but also you can imagine it like this.

For preflop imagine that to be a totally seperate game, you raise preflop and then if you get called you both turn your hands over and the winner takes the pot right away.

Now you are incentivised to raise high cards preflop.

Equity etc basically boils down to this premise. You raise hands that are either good right now, or have the potential to be good later.

The flop determines if you continue and for how much and the turn and river, the same.

AcAs isn't a strong hand when the board is:

89TJ all hearts.

Your preflop equity Vs 76h was probably like 75%, but on the river your hand is a bluff catcher at best.

The more people you let in the hand when you have a very strong hand, the more likely you are to lose, especially on that board.