When you open a player profile, it should look something like this (hopefully, unless you open Minesweeper).
The second block in the top row has the defensive ratings. These show the player's abilities (range, error, arm) and how these translate into positions (with experience factoring into that, however). So, Clyde Brady here has decent defensive abilities, between 11 and 13 on a 20 scale. He is quite good in leftfield, a bit less so in rightfield, and not really good in centerfield. He has appeared at first base at some point in his life.
He is weaker in right than in left because his arm is only an 11. Rightfield demands a better arm than leftfield. The centerfield rating is only a 5, but from his abilities it should be a bit higher, maybe an 8, but he has never really played a lot there.
It works similarly for infielders, but they also have a double play rating. Primarily, middle infielders (second baseman and shortstop) should have high range and high double play ratings. Being sure-handed (errors) helps a good deal. The arm rating works like between left- and rightfielders. Since most of the time on a ground ball, the out has to be made at first base, the further a player is positioned away from first base, the better his arm needs to be.
So, Yoshi Yamada here is a quirky little guy and fantastic defensive shortstop (defensive, since he can't hit a lick). He would be about as adept at second base if he had more experience there. Despite high range, which is also important at third base (the "hot corner"), he has only an 11 arm, and that is not enough to perform very well at the position. He would be able to hold down first base as well, but he has no experience there and you want other players to play first base anyway.
Catchers should just catch and be left alone.
There's a thing to be said about batting ratings as well, but generally, the primary position set by the game is often not far from the best option. F.e., the game will not put an immobile, clumsy first baseman into centerfield.
Also, at times you will have to make compromises. Yamada here is a compromise, playing shortstop every day despite pathetic batting for being a minimum salary player for a team that was never going to do anything but finish last anyway. He can also run like the devil and is in the hunt for the single season stolen base record in this league.
However, while Yamada is a perfect fit for my pathetic Raccoons, he would at best be a backup player on a team actually intent on winning games. If you can find a guy that bats .300 and is only a 12 shortstop, take him, it will get you further, but of course that .300 batting shortstop will cost more money etc....
There is rarely an easy answer in baseball.
