Translation:What do you need?
Much like "vous" in French and "Sie" in German. Used when you meet a stranger, your customer or need to show common respect (like in ads, notices, announcements, etc.).
您 comes from the liaison of 你们, so technically there should be no 您们, In real life, the plural form of 您 is occasionally seen. But more often 您 is used for addressing a single person (usually the customer or any random reader of your sign). For example, it would be weird to use 您们 during a lecture, 各位听众 (or just 各位) would be more appropriate.
There are some other cases where you shouldn't use 您. For example, the author of a book shouldn't address the reader as 您, unless you are trying to express "this book is written specially for YOU!", which is too flattering. 您 is also never used between lovers and relatives, as it gives a sense of aloofness.
Note: It's okay if you can't remember all the nuances. Even if you don't use 您 when you meet a stranger, you won't be seen as too rude, especially if you are not a native Chinese speaker.
Just a though : to me 您 shows respect, the more os when using 需 with 要. Is it correct then to use "What do you need" alone as a translation? I mean, the thing is the absence of the same amount of politeness in the english translation. Is it because I am not a native english speaker that I see things that way or indeed does it lacks a bit of respectfulness in the english sentence to match the chinese one?
The English sentence does lack the level of respect indicated in the Chinese sentence; however, I can't really think of a sentence that conveys the exact same meaning with a higher level of respect. The closest thing would probably be, "Do you need anything?" but I also understand that the Chinese translation for that would be 您需要什么吗 which is slightly different.