If you don't understand exactly what they said ask questions and get them to go over things. Summarise what they have told you. Another suggestion was to literally take notes, whenever practicable. This was also recommended as it tends to build up the mental note-taking habit. Acquiring an understanding of non-verbal communication was recommended.
People say so much, their gestures, looks, and body language in general.
Even the way they build sentences is different when they are happy or sad. Sometimes it is obvious, sometimes you have to know them for a while. That's one part of it. The other is using this information. There's a good way to know if someone is ready for your friendship.