Standard Tuning on a 6 string guitar looks like:
E on the 6th string (bass string), then A on the 5th, then D on the 4th, then G on the 3rd, then B on the 2nd, then E again on the 1st, or top string.
In standard tuning most of the strings are tuned a perfect 4th higher than the one below. The exception is the B string which is tuned a major third higher than the G string below it.
As you know the note a perfect fourth away from the open string will be on the 5th fret so the note on the 5th fret E string is the same as the open A on the next string. When we count up 4 from A we get to D which is the next open string etc. The only exception is that the B string is the same as 4th fret on the G below it.
These are important considerations when converting 6 string tab to 3 string, especially when looking at chord fragments and double stops. More on that later.