Pioneers perturbation theory

Can you please explain what is "pioneers" here? I understand it's a verb but they way they say it, for the moment doesn't make any sense... thanks

Now we get to Laplace, "Celestial Mechanics". Laplace looked at Newton's problem and said: "That's kind of cool! I wonder if I can solve that? This multi-body problem where everybody's tugging on everybody else, but there's one main force of gravity at work". So, he writes a five volume tone called "Celestial Mechanics", and in there he pioneers perturbation theory, that's the theory we have one main force and in other little tugs, how do you treat that mathematically?