Rachel is an unseen character in the television adaption of The Handmaid's Tale. She is either a Martha working at the bar in Jezebel's with contacts to Mayday, or her name is just a codeword for the Mayday contact itself. She never appears on-screen.
In the TV series Edit
Offred (June) reaches out to Alma to become a part of "Mayday" and contribute to the resistance movement. Alma resists at first but ultimately accepts Offred's offer and gives her her first assignment - get a mystery package from a "Rachel at the bar" in the Jezebel's club and hold on to it until further notice. 
Offred isn't able to retrieve the package when she's at the club. She asks Moira to get Rachel's Package for her, which she refuses, claiming not to know Rachel. The next day, Offred is in the supermarket where she orders some meat. The butcher slyly gives her the secret package that she was supposed to retrieve at Jezebel's, with a note from Moira attached. 
When meeting Beth in the Jezebel's kitchen. Nick wants to know if she "asked around at the bar" that "nobody's planning anything stupid", and Beth replies that the girls in here are not "in the shape for a rebellion". This indicates that both Nick and Beth know of a rebel contact at the bar (and that Beth knows who Rachel is). 
Rachel is the only (first) name being used for more than one person within the story.