An ESL is a relationship like one you would have had in elementary school. The couple probably passed notes, didn't talk very much in person and it's based on crushes who have confirmed they like each other.
Sometimes they hold hands, hug and maybe kiss, mostly on the cheek, head or hand. Otherwise there's not much physical contact.
If someone is in an ESL it means they're in a relationship that isn't going anywhere
They're in the very early stages of a relationship.
"Sam and Rachel just started dating. So far their in an ESR but im sure they will get to the action in about a week."
"Kelsey and Joe have been dating for 2 weeks already, they should have more than an ESR."