Quelqu is the abbreviation of the indefinite adjective quelque. It is never used on its own. It is only¹ used in quelqu’un and quelqu’une, the contracted forms of quelque un and quelque une – contraction is mandatory here, you can never write or say quelque un.

Quelqu’un means “someone”. “Quelque un = some one”, if you will. It can either be used to designate some indeterminate person (Il y a quelqu’un ? = “Is someone there?”), or to designate a determined person (C’est quelqu’un de bien = “She/he is a good person”, literally “This is someone good.”). You cannot use quelqu’une for this, even if the person is feminine.

Quelqu’un and quelqu’une can be used to designate an indeterminate person among others. For example Quelqu’une de ces personnes est coupable = “Someone among these people is guilty”.

¹ References: Druide, OQLF, Reverso, Études littéraires.

Quelqu’un is “someone” or “some one [of a set of something, usually people].

Quelque (ou quelques) is just “some” or “several”, e.g. depuis quelques années, j’ai voulu faire quelque chose de passionnant, mais je suis resté ennuyeusement ennuyeux.

As noted elsewhere, quelqu seul (privé l’e) n’est jamais correct.