Although there are many rules to baseball, in general the team playing offense tries to score runs by batting balls into the field that enable runners to make a complete circuit of the four bases.
A player who lacks the offensive or defensive skills needed to be a member of the starting lineup, but who has the various skills needed to play two or more infield positions competently and therefore can be called upon to come off the bench and fill a variety of defensive roles is called a utility infielder.
A player who is tall and has long arms may be a good candidate for first baseman because these attributes help him handle off-target throws.
The first baseman must be able to cleanly field thrown balls that hit the ground before they reach first base, and to quickly decide to abandon first when necessary to catch an especially bad throw.
In some cases, an aging third baseman or outfielder who has lost some speed but is still a good hitter will be moved to first base to keep his bat in the line-up.