There were a few titters in the crowd when they heard the awkward joke.

titters 是动词 titter 的名词复数形式。titter 指"发出咯咯的轻笑"，通常用来形容因为尴尬、紧张或觉得某事有趣但又不适合大笑时发出的那种压低声音的笑声。titters 在这里指"几声窃笑"。