Donate is an activity that you can give some card (Common and Rare) to a clanmate..
You can do it minimal if you're level 3, because it's the level that's unlocks Clan.
Donate can gives you more golds and experiences, and it's good for your economy (for buying something and/or upgrade your cards), and your level (can up your level).
Donate are very important, you can get gems from the donate's achievement when you completed it. Also can up your level because it's important to your Crown Tower's DPS and Hit Point.
The cards that can be donate is Common and Rare only.
If you donate Common cards, you can donate 2 cards and the Experience (xp) is 2 (1 xp/card). But if you donate Rare cards, it can give you 10 xp. But you only can donate 1 cards because of its donate cap. In Common card, its donate cap is 10, but you can only donate 2 card.
Only Common cards and Rare cards that can be donated.