Honor is a currency that you can earn by participating in guild battles and arenas.
The place where you can spend your collected Honor is the guild shop Arsenal. To get to Arsenal, simply tap on the icon of the little tent at any screen in the guild menu or on the button "Arsenal" on the "My guild" screen.
Please note that all the collected Honor will be lost in 5 hours if you leave or get kicked out of a guild. Also, you will lose the Honor if you change the guild.