Picture a hard drive as nothing more than a super long line of mailboxes, each mailbox can hold anywhere from 1000-4000 envelopes.But, these boxes arent numbered when its first setup(like an unformatted drive), so you have to number them into groups so you can store stuff where you can find it again, but each numbered box cant hold mail,so you lose some mail boxes, same with the hard drive, when the filesystem is indexed, the spots marking each block cant hold data, they get changed into indexes or inodes.On a huge drive, theres a whole bunch of inodes, one for each 1k to 4k of disk space depending on filesystem.