sweet and low

Hi. Do you like Friends? I do, and I watch it to keep up my English. In the first episode of the first series, when Rachel meets the other guys for the first time she walk into the Central Perk cafe in her wedding dress and Monica sits her down. While she is starting to explain what happened a watress brings her a coffee. As takes the coffee from the waitress Rachel says: "Sweet and low". Or something like that.
What does she mean by that? Can you explain please? I am not quite sure whether it is a statement or a question but I think it might be a statement as she is from a posh family and probably used to bossing servants around. I think, by "sweet" she might be asking the waitress to put sugar in her cofffee. Not quite sure though, and I am absolutely lost with "low". Thank you.