param is class member of Guard and Controller. It is used to extract route data for placeholder based routing.

e.g - consider the route - "abc.com/{value}/hi" . Here value is a placeholder and this can be extracted using param.

export class FileController extends Controller {

    @Route("/scripts/{file}.js")
    @Worker()
    async getScripts() {
        const fileName = this.param.file;
    }
}

For more info about routing, please check the route doc.