Basically devotion is related to player xp, which in turn is gained every game. At 100 player levels, you gain one devotion level, which will then reset your player level back to 0, and increase the amount of xp you need to gain another level, as well as the iridescent shards you gain with that xp.

Do you get anything for devotion DBD?

Once you have a Devotion level, you get no extra shards for having that 'devotion'. A guide on steam tells me that by the end of the 100 levels, you will have gotten 353,370 XP (dasmaffmaffs guide on steam) and only gained 25,270 shards.

What is devotion in DBD?

Devotion just means you've hit Player Level 100. You go right back to 1 and do it all over again.