The positions are to do with the range between opponents.
The first position is a "melee" (hand to hand) range.
Second position is further back and is considered "ranged".
You can still attack with a melee unit in second position if they have the "reach" ability (green hand).
Third position and beyond is all "ranged".

Units with the ranged attack cannot attack in first position unless they have a special ability to do so, or there is a rule set that allows it.

Units with magic attack can attack from any position.

So, I normally go melee, reach, ranged, magic. But it depends on the rule set.

This video will be a series for beginners, by beginners, made by some Splinterlands employees.