Daydreams are dreams from the Disney/Pixar film, Inside Out. It's where Riley thinks about things other that what is happening. They can also be used as an imagination tool, for seeing things she has not seen yet, or for seeing where things will go.
For example, when Riley is close to the new house, Joy lists the "top 5 choices" of daydreams of what her new home is like. Some of the notable ones are a gingerbread house and a complex filled with slides.