You may have noticed when super cruising to a station or planet, or anything really that you target in the nav map and fly to, that if you are flying too fast when you get close you overshoot and have to do a loop of shame, or if you go the other way and over compensate reducing throttle too soon you will probably end up taking longer than optimum to reach to the drop out zone? Well, the 7 second rule solves this
Simply put, to fly from point A to point B in as optimum a time as possible in super cruise, target point B using the nav map, galaxy map, target button or whatever, set throttle to maximum 100% and keep an eye on the time to reach the target which is in the centre of the HUD, it will be counting down. As soon as it reaches 7 seconds to go, throttle down to exactly 75% and the ship will start to slow down at just the correct rate such that you get to the target in the optimum time without ever overshooting. Don't ask me why 75% is the figure, or why it has to be 7 seconds, or even why it does this, it just does. I am sure there is some physics explanation why but I never bothered to find out. You can push it to 6 seconds usually and still not overshoot, but 7 seconds is the safe number.
The other thing you may see when flying around is the text "SLOW DOWN" in red text in the HUD. This does not mean you are flying too fast and are being told to slow down, it actually means some nearby planet's gravity well is slowing you down, and you can safely ignore the message if you are just traveling along and can keep 100% throttle going without any issue. Its just another one of those things you would never be able to find out in game what it meant and have to hear it by word of mouth. There are some tricks with the slow down thing you can use to break the 7 second rule, but these are more advanced and I personally never bothered to find out.
Edit: It occurs to me you may actually be using the super cruise assist module on your ship. If you do then you won't have noticed the overshoot problem as the throttle management to 75% is what the assist is doing. But once you get other ships and want something else in the module slot instead of super cruise assist, then you need the 7 second rule for sure.