Used to introduce a concise summary or conclusion, often at the beginning of a sentence. Slightly more formal than 要するに.
In short, the plan was a failure.
To put it briefly, he doesn't agree.
Composed of 手短に (briefly, concisely) + 言えば (conditional form of 言う, 'to say'). The phrase literally means 'if said briefly' and functions as a set expression.