How do you understand how to play?
Trial and error. Watching other people play. Using the wiki when I have questions.
Start with the tutorial. It does take some time, but it works you through the basic gameplay rules, step by step. If I recall correctly, every new account is forced through the tutorial. But if you want to refresh your memory, you can re-play it by clicking “Hearthstone” on the starting screen, then clicking the large question mark at the bottom of the screen, and then clicking “Play Tutorial”.
Many newer mechanics are not explained in the tutorial. So the second part of learning is to read card texts, and if you see a bold term that you don’t recognize, hover the card and read the tooltip that pops up.
And third is just practice. As Skizzy already said, play lots of games, watch others play (gameplay streaming sites such as Twitch are ideal for this), and don’t hesitate to ask questions here if you run into something you don’t understand.
Good luck, and have fun!
I’d like to add talking out your plays to yourself.
That helps me out a lot.