d20 refers to a 20 sided dice. The usage of this abbreviation is generally in terms of rolling a 20 sided dice with a prefix that explains how many 20 sided dice to roll. For example 1d20 means roll one twenty sided dice. 8d20 means to roll eight twenty sided dice.
This nomenclature can be used in conjunction with /roll command to have the game interface generate random numbers.
The d20 system is also the set of rules DDO is based on.