Literally: one – sphere – enter / insert – soul
Alternately: This phrase from the world of baseball refers to putting your full energy and focus into each action; specifically, putting everything you can into each throw of the ball.
Notes: This is a compound of compounds. 一球 is a simple number-noun phrase, of course, but 入魂 is more interesting; pronounced jukon (or jikkon or jukkon), it means “intimacy,” “familiarity.” Even as a term for “putting a soul into something,” it can refer to an act that ensouls a physical object, cf. 画竜点睛 and 仏作って魂入れず.