Litotes is a figure of speech in which the speaker expresses a strong positive meaning by negating a word or phrase that means the opposite of the meaning that he intends. For example, someone could intend to communicate that something is extremely good by describing it as “not bad.” The difference between a litotes and a [double negative](../figs-doublenegatives/01.md) is that a litotes heightens the positive meaning beyond what a plain positive statement would do, and a double negative does not. In the example above, the literal meaning of "not bad," taken as a plain double negative, would be "acceptable" or even "good." But if the speaker intended it as a litotes, then the meaning is "very good" or "extremely good."
Some languages do not use litotes. People who speak those languages might not understand that a statement using litotes actually strengthens the positive meaning. Instead, they might think that it weakens or even cancels the positive meaning.
By using litotes, Luke emphasized that there was a **lot** of excitement or anxiety among the soldiers about what happened to Peter. (Peter had been in prison, and even though there were soldiers guarding him, he escaped when an angel let him out. So they were very agitated.)