There is a history to this but as this is a tip I won't go over it.
If you like to read more into it see here (external link).
In short, decimal prefixes are one's you are probably used to, where they're equal to 103n where n is a positive integer that increases by one for each prefix.
Binary prefixes, on the other hand are equal to 210n.
Binary prefixes are similar to decimal ones however their last two letters are replaced with "bi" (for example kilobyte becomes kibibyte).
Here's a table of some values to show the difference.
| n | Decimal prefix | Decimal prefix value (103n) | Binary prefix | Binary prefix value (210n) |
|---|---|---|---|---|
| 1 | kilo (k) | 103 (1000) | kibi (ki) | 210 (1024) |
| 2 | mega (M) | 106 (1000000) | mebi (Mi) | 220 (1048576) |
| 3 | giga (G) | 109 (1000000000) | gibi (Gi) | 230 (1073741824) |
| 4 | tera (T) | 1012 (1000000000000) | tebi (Ti) | 240 (1099511627776) |