I can only definitively identify if someone is expressing a positive or negative emotion, but beyond that I'm just guessing, unless it's very obvious.
Of course, if someone says, "I'm mad," I'm sure we can all understand that because we know the word and what it describes. I'm guessing they mean, "Difficulty understanding others' feelings without having them explained," or something like that. So basically receiving an explanation through their body language, facial expression, tone of voice, etc.
I try to practice, but it's hard because I think it's a thing that not everyone is honest about their feelings if you ask them directly. Right? There have been times I've insisted because I was so sure, such as with a student who seems to be in a bad mood, and I ask how they're doing and they say good, and I ask if they're tired and they say no, and I ask what's wrong, and they say nothing, and I ask if they're sure, they say they are, and I tell them it seems like something is wrong, and they say there isn't.
Then it makes me confused if I'm at all capable of reading any signals at all! *goes mad*
Anyway, re-reading my post, it sounds like maybe it's exactly what they meant in their silly little sentence.