Coming from the world of electric, I’ll explain using frets.
And please beware of @John_E 's post above, as it is not correct. He has the right idea and points out a good piece of source material, but the details are off.
Remember that in upright bass, you only ever cover a 3 fret range with your fingers.
Your first finger, (one fret) your second finger (next fret) and your third and fourth fingers come down together (for the the 3rd fret).
With a fingering position and technique that only covers a 3 fret (3 half step) range, and without any fret indicators on the neck, we upright players need a way to talk about where we are on the instrument.
So the positions.
On upright you start in half position - the open string, first finger on 1st fret, 2nd finger on 2nd fret and third/fourth fingers to get the third fret.
When you move up one half step, you enter 1st position:
first finger on 2nd fret, 2nd finger on 3rd fret, 3rd/4th fingers to get 4th fret.
The positions continue on up the fingerboard, and they don’t move in any logical way as far as I can tell.
Half position and first position make logical sense because it seems like the positions are named based on how many half steps above the open string you are…
But then you move up another half step the the 2nd position, and you’re covering:
1st finger on 3rd fret, 2nd finger on 4th fret, 3rd/4th fingers on 5th fret.
And the logic kinda breaks down, and you just have to memorize the position numbers.
The half step beyond that Simandl calls an intermediate position between 2 and 3… so, again, it’s weird.
Simandl’s method was my source book for upright fingerings.
I think the labeling system is dumb and should be re-done, and maybe there’s a cool new book (as in, any book written after 1910…) that has a better methodology.
Hope that helps.