What I have been told is V6(over)4 to 5(over)3 means it's really I 6(over)4 to a root position dominant if I wasn't mistaken when taking notes? What is this, what does the 5(over)3 mean, why is this more useful than writing the chords flat out, and what purpose does this (I believe it's a part of the dominant function, so function?) serve?
You're talking about what is known as a cadential 6/4 chord, which is a very common embellishment of the V chord. The fifth scale degree is in the bass, but, at first, there's a sixth and a dissonant fourth above the bass which want to resolve to the normal fifth and third respectively (the fourth almost always literally resolves to the third, the sixth sometimes moves differently, like to a seventh above the bass).
You're right that, technically, this harmony has the notes of a I chord in second inversion, and some theorists and textbooks label it as a I6/4. However, most theorists these days prefer for the Roman numerals to indicate the chord's function more than simply its note content. The cadential 6/4 has no functional relationship to a I chord, which is tonic; it has dominant function because actually it's just a dissonant embellishment of the V chord. As such, most textbooks and theorists I know tend to prefer to label it as a V chord in order to show its function.
Whether this is the best possible choice is up for debate, but that's only a symbolism debate. Everyone agrees about the actual function of the chord. Often, composers of the Classical and Romantic periods treated the cadential 6/4 as the primary dissonance in the underlying harmonic structure of their music. For example, the unstable harmony that introduces and (at least theoretically) undergirds cadenzas in concertos is almost always a cadential 6/4. The trill signaling the end of the cadenza tends to mark the resolution of the sixth and the fourth to the fifth and the third above the fifth scale degree in the bass, which then leads to a PAC on I in most cases.