This revolves around a couple of related idioms, actually, related to baseball and softball.
Let's say two people are engaged in a debate. One says something that makes it easy for the other to make an incontrovertible point, and win that round of the debate. In the debating circles, the act of making a memorable, incontrovertible point is sometimes called "hitting a home run," and the gaffe that makes it easy for that home run to be hit is sometimes called "an easy pitch" (or "a softball"). One renowned softball I remember was when Dan Quayle spoke of how his relatively young age shouldn't be in issue in him being qualified to serve as president, if needed. His opponent, Lloyd Bentsen, was ready for that pitch.
Sometimes this baseball/softball analogy is applied to interview situations as well. If a journalist giving a recorded interview doesn't ask any hard questions ("hard" in this case means something that would make the interviewee uncomfortable, such as questions about a recent scandal, perhaps), that is sometimes described as softball questions, or a softball interview (see this headline, for example: Kroft's Softball Obama Interviews Diminish '60 Minutes'). In a similar way, an interviewer who asks an unexpectedly difficult question might be said to have "thrown a curve ball," or one who asks a lot of tough questions relentlessly might be described as "playing hardball" during the interview.