Every XRP ledger account has a 20 XRP reserve to activate the account. There are other reserves as well.
The XRP Ledger applies reserve requirements, in XRP, to protect the shared global ledger from growing excessively large as the result of spam or malicious usage.
To submit transactions, an address must hold a minimum amount of XRP. You cannot send this XRP to other addresses. To fund a new account, you must send enough XRP to meet the reserve requirement.
The current minimum reserve requirement is 20 XRP. Each new account must set aside this much XRP, which cannot be recovered or sent to others.
The 20 XRP reserve is not a requirement by XUMM, but a characteristic of the XRP ledger.
Other transactions can cause the reserve to become higher, eg.
An open offer on the decentralized exchange
Added currencies (IOU's, Trust Lines)
They will add 5 XRP to your account reserve. Your account with 2 added currencies (eg. Bitstamp USD and Gatehub EUR) will thus have a 30 XRP reserve.
When finishing an escrow, when deleting or fulfilling your offer on the decentralized exchange or when removing your added currency, the reserved XRP will become available again.