An alt is an alternate account, as opposed to a main account(the account that you dedicate all your time/money on).
Alts are generally made on the same server as the main account for several reasons which I shall not mention due to the fact that they are mostly against the rules. Suffice it to say that generally, these alts are used to benefit their main account which, if caught by support, will land you in a pot of hot water. If you come across anyone admitting to it, do all the fair players out there a favour and screenshot it and email it to
kingdomsupport@haypi.com.