The level up page allows you to move to the next level and receive the rewards for doing so.
The first drop down is the level that you are moving too.
The next 3 drop downs allow you to configure your level badge. The first one allows you to set the image you wish to use. The second one allows you to change the border colour. The final allows you to change the text position.
The next set of drop downs are for your choice cards.You can either select the deck and card manually, or select the collecting deck link which will automatically set them for you.
If you are mastering the deck you currently have set as your collecting deck, you can use the final drop down to change this deck.
Once you are happy, select level up and it will present you with your cards.