Translation:I understand you.
It is bad enough that we have every possible combination and permutation of I/we/you/you all/it/them/he/she/her/him/they/me/us to I/we/you/you all/it/them/he/she/her/him/they/me/us and vice versa, but we already have this I to you prefix, qa, why make up a duplicate Sa? Just to make it harder?
No, it's not just to make things harder. It's to specify the "you" being addressed is a group. This helps clarify things, for example, when plural suffixes are omitted.
Modern English is unusual among languages for not having a plural you. It used to.
Right. The transition is actually quite interesting.
English had thou for singular, second-person subject, thee for singular, second-person object, ye for plural, second-person subject, and you for plural, second-person object.
Later, the plural forms came to be used singularly to indicate formality, much as a monarch might use the first-person plural to refer to him- or herself ("We are not amused.") You would replace thou and thee to indicate formality. Spanish, for instance, has such a distinction, with tú being informal and usted being formal, and usted grammatically treated as third-person. ("Would sir like to see the wine list?")
(You can see a similar process happening now, where the third-person plurals, they and them, are being used as a gender-neutral third-person singular. This usage is being considered more and more proper, to the point that some important style guides are incorporating singular they into themselves.)
Eventually, the formality divide disappeared in English, leaving the previously formal you as the dominant form of the second-person in all uses. You only see thee, thou, and ye in archaic English.