What it stands for
Mercury covers how you take information in and hand it back. It’s the difference between someone who thinks out loud and someone who disappears for an hour and returns with a finished conclusion. It covers humor, curiosity, how much detail you want in a story, and whether you read the instructions.
Because Mercury never strays far from the Sun, it’s always in your Sun sign or one of the signs on either side. Two people with the same Sun can still have quite different minds.
Where it shows up between you
The Communication area of a report is tied to Mercury, and its effects are usually easy to spot. A contact with their Moon can decide whether a hard conversation feels safe. A contact with their Venus often sounds like flirting by text and pet names that stick. A contact with their Mars can be sharp debate that’s good fun until one of you stops having fun and the other hasn’t noticed.
A challenging contact doesn’t mean you can’t talk. More often, you’re each answering a slightly different question. One person hears “we should leave by six” as a plan, and the other hears it as criticism of last time.
Something to try
Before a difficult talk, ask which they’d like first: solutions, or listening. It feels a bit formal at first, but it heads off a very common fight, the one where someone offers a fix to a person who wanted company. Also notice how each of you prefers to disagree. Some people do it best in writing, or walking side by side instead of face to face. There’s no wrong format if it gets you to the end of the sentence.