It is used to express readiness and motivation to begin something. It signals that the speaker wants to start an activity immediately. The structure 'get this on' can also be used informally to mean put this on when referring to clothing. It describes the physical action of dressing. It is not the same as the expression let’s get this on, which means let’s begin. For clothing, it simply describes putting a garment on your body. Imagine a mother helping her very small child get ready for school. "Let's get this sweater on before we leave."