Here hot means a “backup” component and spare means extra disk which can be used when a disk gets failed.
So hot spare means a backup device.
Hot spare is also known as warm spare or hot standby.
Hot spares word is related with RAID technology.
Read about RAID here….
In RAID, multiple disk are connected together with same data content, when a disk gets failed another disk get activated with same data and same operation.
Hot sparing is the substitution of one disk or drive for a failing or failed disk or drive.
A hot spare, also called a hot standby, is a backup component that can be placed into service immediately when a primary component fails.
When one of disks fails in a RAID group, than hot spare disk will replace the faulty one, and data will be rebuilt to the hot spare.
In RAID arrays one disk is used as spare to rebuild the system as soon as a disk fails.