Topup is a function of Gitmark that lets you increase the amount of funds in a commitment. This allows a repo to continue operating after it has become low on funds.
Simply add another tx input to your transaction, and the new transaction output will have more funds in it.
Certain versions of the software may incur donations; this, in turn, could be used to top up the project and keep it going longer.
Topup is a super simple workflow that sweeps inputs into a new output spliced together with a mark, in order to increase funds on-chain.