Dogs use a behavior called a play bow to signal their desire to engage in, or continue, play with another dog, human, or animal of another species.
The leading animal scientist Marc Bekoff describes the appearance of the play bow:
When performing a bow, an individual crouches on its forelimbs, remains standing on its hindlegs, and may wag its tail and bark. The bow is a stable posture from which the animal can move easily in many directions, allows the individual to stretch its muscles before and while engaging in play, and places the head of the bower below another animal in a non-threatening position.
The play bow is said to be stereotypical, because it always looks the same, and does not appear to be learnt; rather, it is an innate part of every dog’s behavioral repertoire.
Bekoff, M and C. Allen (1997) Intentional Communication and Social Play: How and Why Animals Negotiate and Agree to Play. Marc Bekoff and John A. Byers (eds.) In: Animal Play: Evolutionary, Comparative, and Ecological Perspectives. Cambridge and New York: Cambridge University Press