Reload bonuses are very similar to welcome bonuses. They are usually given at a lower percentage (less than 100%) and to existing customers only. So, for example, a happy hour reload bonus offer might give you 30% on every deposit.
They can also be used to reactivate players who have not been active for a while, or then to reward active players. This is one of the most common ways that casinos reward existing players.
Deposit 30€ and play with 60€ this means that when you deposit 30€ the casino will match your deposit.
Deposit 100€ and play with 150€ this means the casino will give you an extra 50€ to play with.
To keep in mind, there is nearly always a wagering requirement on the bonus or then it can be on both, the bonus and the actual deposit.