The documentation says
The executor to use to launch this application.
Different executors are available.
The simplest one (and the one configured by default if none is given) is `//cmd`,
which takes the cmd and executes that on the shell level.
It does not say what an executor is or what executors are available. See https://github.com/mesosphere/marathon/blob/master/docs/docs/rest-api/public/api/v2/types/app.raml#L181.