For every Marker increase, you need to consume Energy. There are 2 ways to replenish your Energy:

1. Wait for it to recover automatically (its recovery rate depends on your current level).

2. Buy Energy using Diamonds. This will increase your Energy Level and also give your character XP.

On the Energy tab, you can see the currently available Energy (the blue box) and the maximum Energy that you can store (the yellow box). Keep in mind that even if you use Diamonds to replenish your Energy, your Energy level can't be higher than the maximum Energy that you can store.