1. To be able to be applied (onto someone or something) as a mist or aerosol spray. This sunblock simply sprays on, making it easy to keep you and your family protected during the summer.
2. To shoot a stream of some mist or liquid onto someone or something. In this usage, a noun or pronoun can be used between "spray" and "on." We had to keep spraying on bug repellant the whole time we were camping. It got so hot that we had to spray water on the road to keep the asphalt from melting.