Use this to interrupt someone's story or explanation. For example:
A: No, you see, I wasn't able to get the curtain rod because the place I went to only had really short ones, and then when I went to the next place —
B: Wait a second. They only had short ones. Like how short?
"Wait a second" is really useful for situations where you don't want someone to continue talking because you're worried that the conversation will move on to another topic, and you'll miss your chance to say something. This is especially useful for asking people questions about something you don't understand:
A: So here's a prescription for your nasal decongestant and for the antibiotics. Now, it's important that you continue taking these until the prescription completely runs out.
B: Wait a second. Are you saying I have to take both of them until they run out, or just the antibiotics.
A: Sorry, yeah. I just meant the antibiotics.