Answer this question, quite briefly.
The memory and disk space statistics are displayed using IEC binary prefixes, KiB, MiB, GiB, TiB (for kibi, mebi, gibi and tebi). These are intended to distinguish binary reporting of sizes used in System Monitor (multiples of 1024) from decimal sizes (multiples of 1000) commonly used, for instance, in packaging of hard disks.
Typical binary units:
1 KiB = 1024 bytes
1 MiB = 1048576 bytes
1 GiB = 1073741842 bytes
An external hard drive, advertised as 1.0 TB (terabytes), would display as 0.909 TiB (tebibytes).