A coaster protects a surface from marks, so dream books read it as small precautions and the care taken not to leave traces. It can point to caution about the impression you make.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.