The term main (also referred to as primary account) usually refers to a player's highest levelled account or their most frequently used account.
This term is usually used by players who have multiple accounts, most commonly referred to as "alts" (short for alternates). These accounts are usually disposable and in essence, they are technically 'pures', as they are created and trained for multiple purpose(s), such as money making and typically, only a specific set of skill(s) are focused on and trained for any gain to the player (usually monetary).
Although it used to be forbidden to log into more than one account simultaneously, this was changed in May 2014 [1][2], allowing trade and other interaction between alternate accounts "as long as it doesn't involve rule breaking or gameplay exploitation".