There are two different storage formats you might see used. If you buy a storage device, such as a hard disk, or a solid state drive, the drive size is in decimal format (base 10). So when the drive manufacturer says you've bought a 1TB drive, they mean 1TB = 1000 bytes x 1000 [into MB] x 1000 [into GB] x 1000 [into TB].
But computer operating systems have traditionally reported storage in binary format (base 2). So users who never received the Great Storage Memo (indeed, even my copy was lost in the mail) would get curmudgeonly, grumpy or otherwise a little stroppy that they were missing out on space they'd purchased.
So, some bright sparks got together and came up with some entirely silly storage names for base-2 units. It's mebibyte instead of megabyte, gibibyte instead of gigabyte, tebibyte instead of terabyte, and so on. And thus, infrastructure and storage consultants get to say nonsensical sounding words in meetings like "tebibytes".
Yay, technology!