You won't see unallocated space in Windows Explorer, only find that a hard drive is showing less space than actual, for example, 1TB shows 700GB. This means that no programs can write data to this space, nor save data until it's being a part of the real disk partitions.
A computer describes any physical space on a hard drive that doesn't belong to a partition as unallocated. Unallocated space, as the name suggests, is part of disk space that is not allocated to any partition.
