What are those numbers on the screen?

There are numbers in the bottom right of my screen.

I do not know yet what to do. I passed the explanations by mistake. I tried to understand by activating, but I'm not sure to understand.

These are your Spares. Spare cards are cards that you can use to change the current card that is available in your foundation deck.

Seeing as you said you didn't quite understand the usage, I'll outline it in images. Having numbers 2 through 10 is the maximum number of spares that you can have at once. If you find another spare of the same number that you have already (whether you are full or not), it will go to waste. I'm going to use my #6 spare card here.

Now you can see that the 6 is no longer available as a spare for me, as I've used it, and a 6 card has appeared on my foundation deck.

Spares will appear randomly underneath cards as you play the game. They are not used until you use them. The best use of them is to increase your combo multiplier, letting you continue to add cards to it without having to break your streak.

If you need a reminder about their functionality, or anything else in the game, you can click on the green "More" button on the left at the Main Menu, and click the blue "Help" button here!

Those numbers are bonus cards. At any time during your game you can substitute your last card put in the deck with one of those bonus card, without resetting your combo counter, thus allowing you bigger scores.