This is a Hubble image of the star Mira.
The midpoint is exactly what it sounds like, the exact middle point between 2 planets. It is important when a planet is at the midpoint. I've read to use a 3 degree orb. If a planet is within 3 degrees of a midpoint then it is at the midpoint. People who are interested in astrology usually want to know about their love life. So the Venus/Mars midpoint is of special interest to them. Venus being the feminine and Mars the masculine principal. You can go to Astro.com and get a natal chart for free and then there is a site on the internet that will calculate midpoints for you. It is easy to do. If Mercury is at your Venus/Mars midpoint then you are always talking about, and thinking about relationships. If Saturn is there then you will have frustrations and difficulties and constraints on your relationships with the opposite sex. Some of us don't have a lot of midpoints in our charts, I think I have only 2. The midpoint of my Mars/Venus is the asteroid Juno. Juno represents our marriage partner. I tended to look at my romantic partners seriously with a thought for the future. I was thinking about possible marriage all the time, or if the relationship had a chance or if I was wasting my time basically.
Noel Tyl writes great stuff about midpoints at his site. The midpoints of these planets can be, he says, interpreted like this:
Saturn + inner planets = Frustration, Inner Criticism or Self-Mastery and Achievement
Mercury or Jupiter + Saturn, Uranus or Pluto equals intellectual gifts, communication skills.
Mercury, Venus or Neptune in any combination equals Idealism and Perfectionism
Saturn + Outer Planets (Uranus, Neptune or Pluto) = Harsh Realities Shape the Life
Of course, when you know very well the meanings of the planets and you know the midpoint is a blending of the 2 energies then understanding will come to you.
I have my Sun at the midpoint between Mercury and Saturn, so communication skills for me!
I have Jupiter at the Sun/Moon midpoint so I'm kind of stuck with positive thinking, like I have to do it, which is somewhat limiting. If I don't think positive I absolutely cannot do anything. I think that is the problem with this midpoint.
Also interesting is that what you talk about is Mercury. It appears that if you look at aspects to Mercury you can see what comes out of your mouth. The only aspect my Mercury has is to Venus, this planet wants everything to be beautiful and pleasing and this is the semi-square. This is not a major aspect but I'm told it counts.
Also interesting are Egyptian bounds, and if I follow my bound Lords I find that Mars is the Lord of both my Sun and my Moon, but Venus is the bound Lord of Mars.
I have found a puzzle to work with in astrology and if I put it all together perhaps I'll see a picture of myself. I find it to be an uplifting thing to do.
There were various "hooks" along the way which hooked me on Astrology. Each thing I learned verified astrology for me, this is true, yes, this is so, and seeing myself in the planets. Chiron and how it represents wounds and how the 3rd house represents siblings and elementary school and how my brother and constant teasing in school hurt me. This is truly my wound. Astrologers say that your wound is how you can help others, that this is your task. I don't know how to do this. Jupiter is telling you where you have the most room to grow by it's house position. Neptune is telling you where you don't see things too clearly. Venus tells you where you love to be, and Uranus tells you where your purpose and talents may lie. Pluto could be telling you where your personal power is. Mercury tells you how you communicate and what you like to talk about. All of this is potentially very useful information and can lead you to think about yourself.
As one of the ancient Greeks said (which one was it?) "The unexamined life is not worth living."