Schematic representation of an exemplary image sequence showing possible inter-frame referencing relationships in temporal ("inter") prediction in VP8:
Predicted frames (marked with letter "p") can copy (motion compensated) image data from other frames and code a difference signal to correct this prediction.
Each predicted frame can use (up to) three buffered reference frames: Its directly preceding frame, the last so-called "golden frame" (depicted in yellow colour), and the latest "alternate reference (altref) frame" (marked with letter "a"). Altref frames may signal to not get displayed after decoding and only replace the altref buffer content.
In asynchronous use cases, the encoder could look ahead and use an evaluated future frame to construct an altref frame from frames that lie ahead in time.
Intra frames (marked "I") don't rely on temporal prediction and hence don't reference any other frame.

Blue arrows signify how image data gets copied over to be used to (re-)construct another frame.
Own work
Flugaal