Once when I was on a plane that was about to land, the cabin crew came around and they were spraying these small canisters. I've been on many flights but it was that particular flight where they did the spray. Can someone please tell me what those spray's are?
The sprays are (usually) insecticides used to prevent the spread of invasive species and diseases.
The process of ‘disinsection’ is required under the International Health Regulations of the World Health Organization (WHO) on flights to and from certain destinations to prevent infectious and contagious diseases carried by insects and other volatile bodies. Rules established by the International Civil Aviation Organization (ICAO) require that disinsection does not injure or cause discomfort to passengers or crew. These rules permit the use of certain insecticides, which have the approval of and are recommended by the WHO based on their effectiveness and safety.
From the US dept. of Transportation
I was on an extremely long flight once, and the cabin crew walked up the aisles around the halfway point spraying canisters (about two in each hand) of what they said were disinfectants, required by the World Health Organization because we had so many people cooped up in a limited-air-exchange environment for so long. The idea as explained was to prevent passenger-to-passenger disease transmission.
However, on looking it up, I find the WHO's page refer to disinfection of aircraft as being primarily about killing insects, so there may have been an error in communication. See also this USA Today article for more information.