A more thorough explanation:

A standby guardian is a person appointed by a parent to take care of their child in the future if the parent becomes unable to do so. This person does not take away the parent's custodial rights. The appointment is usually made in case the parent becomes terminally ill or incapacitated.

For example, if a single parent becomes terminally ill, they may appoint a standby guardian to take care of their child after their death. The standby guardian will assume responsibility for the child during periods of the parent's incapacity and upon the parent's death.