1.
fastball, straight pitch
In baseball, a pitch thrown straight without spin or break. The most basic type of pitch.
2.
directness, being straightforward, bluntness
A figurative extension from baseball: saying or doing something in a direct, unambiguous way without beating around the bush.
Usage
Originally a baseball term, but the figurative sense 2 (directness, bluntness) is extremely common in everyday conversation. The na-adjective-like usage 直球な is widespread in colloquial speech. The adverbial 直球で means "straightforwardly" or "point-blank."
Writing
直 means "straight" and 球 means "ball." The compound literally means "straight ball."
Similar words
変化球 (breaking ball, curveball) is the opposite in both baseball and figurative usage, meaning an indirect or unexpected approach.