What do the units mean?

DC++ has always measured file sizes and speeds with the understanding that 1024 of something make the next unit up. Using SI prefixes, kilo means 1000, so one kilobyte should imply 1000 bytes. In popular usage, one kilobyte nearly always means 1024 bytes, which means that "kilo" has multiple definitions depending on context. The International Electrotechnical Commission (IEC) created new prefixes to remove the ambiguity, so kilo could always mean 1000 (and so on). In v0.402, DC++ adopted these prefixes.

New Prefix Factor Old Prefix Factor
kibi- 210 kilo- 103
mebi- 220 mega- 106
gibi- 230 giga- 109
tebi- 240 tera- 1012
pebi- 250 peta- 1015

For more information about the binary units see the National Institute of Standards and Technology.