The term "single layer" means that there is only one level or thickness of something, without any additional layers on top or underneath it. Full definition
Toss together figs, honey, vinegar, oil and sugar; arrange in single layer in small baking dish. (fishernuts.com)
Meanwhile, place sweet potatoes in bowl, drizzle with 1 tablespoon olive oil and spread in single layer on prepared baking sheet. (connoisseurusveg.com)
A prominent example of this is graphene, which is made of single layers of carbon atoms. (sciencedaily.com)