All can translate to "some" but often with nuances:

Certains means "some" as a subset of a group.
Certains ne sont pas venus. → Some didn’t show up.

Des is the plural of un, some as "more than one".
Des garçons jouent. → Some boys are playing

Quelque is a singular "some", often like "any". It is literary except in some fixed expressions like quelque chose (something/anything), quelque part (somewhere), quelqu’un (someone/anyone).

C’est une chose admirable, que tous les grands hommes ont toujours du caprice, quelque petit grain de folie mêlé à leur science, Molière, Le médecin malgré lui.

It is an admirable thing, that all great men always have whims, some small touch of madness mixed with their wisdom.

Quelques means some like "a few", "a couple", "a handful".
Je reviens dans quelques jours. → I’ll be back in a few days.

Quelques-uns means some like "some of them", it is close to certains. Note that quelques-uns can refer to people, things or whatever and has a regular feminine form (quelques-unes) while quelqu’un is necessarily a person and is very rarely used at the feminine (quelqu’une).
Il y en a quelques-unes de cachées → Some of them are hidden.

Pas mal de is a "bigger some" like "quite a bit", "quite a lot".
J’ai raté pas mal de choses. → I missed quite a lot of things.

Certains means some people or a few: "Certains anglais parlent Français" may translates to "Some english people speak French". Without the s, it indicates when someone is sure about something "J’en suis certain qu’il est fort" translates to " I am sure that he is strong".

Quelque is similar to certains but is generally more vague like Je possède quelques livres translates I own a few books.

Quelques-uns which is the plural form of quelqu’un meaning someone.

Pas mal de means a fair amount like Je suis allé au match et il y avait pas mal de gens translated to I went to see the game and there was a fair amount of people.