Resolution says the dimension of the targets that you can detect at which distance.
Let’s try an example.
Go to the window “Create Research Project”, and select the “Active Sensors”.
Set the Sensor Strength 10, then EM Sensor Sensitivity 5. Don’t touch the other parameters.
In the right panel of the window, you should find these rows:
Resolution 100 Maximum Range vs 5000 ton object (or larger): 18,517,239 km
Range vs 1000 ton object: 740,690 km
Range vs 250 ton object: 46,293 km
Now, back to the left panel.
Try to change from “Resolution: 5,000 (100 HS)” to “Resolution: 1,000 (20 HS)”.
On the right now you get
Resolution 20 Maximum Range vs 1000 ton object (or larger): 10,828,947 km
Range vs 1000 ton object: 10,828,947 km
Range vs 250 ton object: 676,809 km
So, you have two sensors, of the same dimensions (1 HS (50 tons)), i.e. mass installed on a ship, but the first (Res 5,000) can see an object of 1,000 tons at 740,690 km, while the second (Res 1,000) can see the same thing of 1,000 tons at 10,828,947 km.
I’m not sure about the exact distance, but the second sensor can surely see a 5000 ton object much, much more far away than the first sensor.
The largest sensor I can build is Size 50, i.e. its mass can be as large as 2,500 tons.
Using the parameters of the first sensor, even if I build a Size 50 sensor, it can see a 1,000 ton object at most at 5,237,466 km = half the distance of the second one.
You can think of this: the technology of the second sensor assures that it has much more power than the first one, so it can detect the objects (also the smaller ones) from a very long distance.