A frame of reference is where you or the given object exists in a three-dimensional plane, by which you or it can observe another object in the same three-dimensional plane.
For example, you are sitting next to the train tracks. Where you are sitting is the frame of reference to which you observe the moving train.
Now say the train moves at 200mph.
If you are sitting, from your frame of reference the train is moving at 200mph.
If you want to get more complicated, the train appears to be coming toward you much slower than 200mph and gradually increasing until it is directly in front of you, which it appears to be moving at 200mph, and as it travels away from your frame of reference it appears to not only be getting smaller but also appears to be slowing down, even though objectively it is not.
However, now say you are in a car parallel to the train and you are moving at 60 mph.
Now, from your frame of reference the train moving at 200mph seems to you to only be moving at 140mph - that is, the moving speed minus your speed.
Here is another example:
You are standing in an airfield. You look up and see a plane just 50 feet above you. From your frame of reference, the plane appears to be moving very fast.
You look up again and see a plane high up at 30,000 feet. Even though that plane is moving at the same speed as the first plane, from your frame of reference it appears to be barely moving at all, because it is so far away.
