My resources are at a cap, what does it mean? What are the limits?

Most resources have a limit on how many you can have in your inventory. You can still play battles normally and you will start gaining the resource again once you have spent some of them so that your resources are below the limit again.

These are the maximum amounts for different resources:

Rune Keys - 1000
Runes - 50
Friendship - 750
Energy Berries - 25
Arena Tickets - 25
Hatch Tickets - 25 (of each type)
Premium 10x Hatch tickets - 10
Dungeon Keys - 75
Totems - 120 (Prestige and Super Shot)
Anger Totems - 100
Evolution Material - 18000 (of each color)
Evolution Essence - 400 (of each color)