They communicate what they feel and know, rather than hiding behind a mask. Candor on the part of the listener tends to evoke candor in the speaker. When one person comes out from behind a facade, the other is more likely to as well. In some cases, the principle of congruence can be at odds with the principles of empathy and acceptance. For example, if thc listener is annoyed with the other person, they probably have to suspend empathy and acceptance until they sort things out.
Concreteness refers to focusing on specifics rather than vague generalities.
The listener can encourage concreteness by asking the speaker to be more specific. In active listening, it is important not only that the listener have an orientation with the four qualities of empathy, acceptance, congruence and acceptance, but that the speaker feel that listener has this orientation. Consequently, a good listener tries to understand how the other is experiencing the interaction and to shape their responses so that other person understands where they are coming from. Furthermore, the listener must be prepared to deviate from the four principles if that's what the other person wants. For example, if the other person asks for an opinion, the listener should give it, rather than avoid it as implied by the principles of empathy and acceptance.