Extracting value from opponents is when you have a good idea, that your hand is the best hand, and you get the most value/chips back when you stack your opponent. You're trying to string them along when they have weak hands, and your hand is stronger without them making a better hand. It's good to practice making value bets, and it varies from player to player, not all players will call the same bets, or fold to them. Sometimes you need to change bet sizes to get value, or increase the likeliness of a fold.
I love trying to get the most out of every pot/hand, and extract value when I bet, the most your opponent is willing to call, or bet less to increase chances of a re raise. It's important to make sizeable bets, but also bet the most your opponent is willing to call when you have a very strong hand. Sometimes you may want to play it safe, and not go for a value bet if you only picked up a little information on your opponent, or are just unsure, play it safe. It's key to know when to make a value bet, and also using pot control is another thing I like to do.
You can use blocker bets which are lower bets to prevent/slow down other players from making bigger bets if you trying to see another card, or cheap showdown. Blocker bets help you conserve your chips which you may need to win a bigger stack in another hand. Also using pot control as I previously mentioned helps you get to showdown for less, and if your chasing and make a stronger hand, you can extract more from your opponents if they don't believe you made a better hand. Checking/Calling is a good way to effectively use pot control when done right. You don't want to give away too many chips.
Every time I make a bet I'm either betting for value, pot control, or trying to take a stab at the pot. I try to keep all my bet sizes the same, and rarely change it up. It's a easy tell when you over bet with a strong hand preflop, and you don't want players to figure out any betting patterns, or pick up information on your game. Only bet what you're willing to rebuild from when sizing your value bets, or use pot control(check or make small bet).