Once upon a time you put a coin in a slot and pulled the handle. If three paying symbols came up in the middle of the window, you won something. That was a one credit, one line, one denomination machine.
Fast forward to video slots. Now you can choose whether to play one credit, max credits, or something in between. That's your bet on each line. Some machines will only pay the jackpot if you're playing max credits. Some just pay the same amount per credit whether you bet one, max, or something else. Max can be as high as 20 coins.
And these machines have multiple win lines. You typically can see there symbols on each "reel". Its all video, but they still simulate spinning reels. You'll see what I mean after watching it a time or two. In addition to winning if the middle symbols line up, you can choose more "lines". Some straight across, some diagonal, some complicated zig zags. Don't worry about the details, the machine will do it. You just have to pick how many lines you want.
Your total bet is then the number of lines times the bet per line. Its not unusual for a "penny" machine to let you bet 20 lines at 20 coins per line, so you're betting $4 per spin. If you want to extend your playing time, 1 coin per line and max lines can be fun. 1 coin and 1 line on these machines is boring.
Some machines let you choose what "a coin" means. These are multi-denmoination. Not too common on slots, but quite common on video poker machines.
Some of those video slots have bonus rounds. If you get a certain combination of symbols, you go to the bonus. Typically they show you a bunch of choices and you choose until you bust out. My wife like All That Glitters. It shows you 20 gems and you pick one for the gem cutter to try to cut. Sometimes he succeeds and you get credits. Sometimes the gem breaks. Break three and you're back to the regular game. All the ones I've seen work in some similar fashion for the bonus rounds. But there are others like the wheel of fortune machine that spin the wheel and drop the ball.