Standard perturbation theory - what exactly is meant?

"standard perturbation theory" - what exactly is meant?

hi,

standard perturbation theory can be viewed as a special case of the
semiclassical method, where one quantises fluctuations around trivial
classical solutions, whereas in soliton or instanton physics, one does the
same thing around non-trivial, non-perturbative classical solutions.

the quantization around trivial classical solutions can be done via the minkowskian path integral, while instanton solutions arise in the euclidean formalism (using a semiclassical approximation, in the sense of stationary phase approximation and steepest descent).

however, i'm not sure what the author (Rajamaran) means with standard perturbation theory and the assertion that instanton solutions are non-perturbative classical solutions.

Thanks a lot.

(i hope my question makes sense, i'm quite new at qm)