Proto-languages are languages for which we have no direct evidence of their existence. That is, there are no inscriptions, scrolls, or anything resembling writing that can be found. In linguistics we reconstruct proto-languages from 'regular correspondences'. Regular correspondences are when pairs of corresponding syllables or speech sounds exist between two languages in words that mean the same thing. For example, 'father' in French is 'père', and in Spanish it is 'padre', while 'mother' is 'mère' and 'madre' respectively. These word pairs can then be used to find a common ancestor of the language. In the case of French and Spanish, we know that these two words come from the Latin words 'pater' and 'mater', but the same is not the case for proto-languages, where we have no direct evidence of their existence.

Another important idea at play here is sound changes. Certain sounds very often change into other particular sounds, which allow you to pin down how exactly a proto-word was pronounced. To use a hypothetical example, let's say you have language A and B, and in language A, all words that start with "p" start with "f" in B, so the word "pik" in A is "fik" in B. We know that "p" very often changes to "f", thus we can conclude that proto-AB had a word "*pik". The same is applicable to all aspects of language including grammar, and from this we can reconstruct proto-languages.

This technique is obviously not 100% accurate, as it's only based on the evidence we have, but we can safely assume that people DID speak something quite similar to the reconstructed proto-language.

EDIT: Wonky accents

While answers are always welcome, I believe this question would be more welcome on /r/linguistics .