In messages, "ATM" stands for "at the moment," which people often use when updating something's current status (similar to rn). For example, your friend might text you if you want to buy tickets to Friday's game, and you respond with, "I'm not sure atm. But I'll let you know tonight."

Most people use ATM when texting, messaging online, or when gaming. For example, if a gamer asks if you are available for a campaign, you might respond, "I'm tied up in another one atm but I'll lyk when it's done." Or, your buddy may ask you how you are doing in a message online, and you respond with, "Not great atm. Been battling the flu. :("

People may also use ATM for "automated teller machine." Typically, they'll use this when speaking about matters related to money.

More Info: slang.net