On the Comixology website (and NOT on the app), you can buy comics for yourself, or there's a button to send as gift. This puts in your cart as a gift. When you check out, it prompts you to put in the recipient's Comixology username and email address (which the recipient of the gift will have PM'ed you). Buy the comics. The recipient gets an email, and when they click the link therein, the codes are added to their account.
If you're using only the Marvel App, then to receive Comixology gifts you'll need to sync your Comixology and Marvel accounts. It's pretty easy; there are instructions on Comixology.com.
It's a great way to trade new comics for back-issues. If I don't have something on someone's want list, but they have something I want, I can offer a gift. The gift is always worth less if you just traded for a code (there's no value in trading a code for a $3.99 gift; why not just buy the book yourself?), but it is a great way to build your back-issue collection. Or to take advantage of sales.