Don't pay attention to the position listed on the player card, you can change those yourself. There are only two things that matter to determine where a player can play defensively. 1- his defensive ratings and 2- does he throw righty or lefty. If he is a lefty, he can only play in the OF or at 1B. If you play a LH in the IF he'll get all kinds of '-' plays.
Next, click on GM Office. Then click on one of your position players positions (I.E. click '2B'). A screen will come up that will allow you to change their primary and secondary positions. More importantly, a chart will pop up showing all of the average ML defensive ratings for each position.
Range and arm strength relate to '+' or '-' plays. I.E. above average range for the position will have more '+' plays and less '-' plays. Below average will result in little or no '+' plays and many '-' plays. Same for arm strength.
Glove and arm accuracy relate to errors. I.E. above average glove or arm accuracy will have fewer errors, while below averge glove and arm accuracy will result in more errors.