I can explain this a bit as I learned a bit of orienteering from youth groups/military experience. So when working with a compass you have to actually modify the declination a bit. All this is rotating the angle markers. To determine that requires a little bit of math. This is because True North location actually moves over the years. So what he's stating is that the bearing to the location has shifted. As if you had a compass set to 0 declination which means 0 points at true north the bearing would be different.

​

Quick explanation on what a bearing is. So when using a map the lines straight north are considered 0 degrees. East is 90, South is 180 and West is 270. So by drawing a line from one point to another you can use a protractor or other angle measuring tool to determine the angle.

​

If the map is new and set properly to true north there is no declination set on the compass. So what you do is you rotate the right so that the angle you determined from the map is set to the front of the compass. You then rotate the compass(typically by moving yourself as you point it outwards from your body) till the north on the magnet and north on the reading align. You then know you are pointing at your bearing.

​

With declination you have to adjust that angle a bit(usually compasses have a key to do it so you don't have to adjust on calculation). So on an old map you calculate declination and then when you shoot your bearing you'll still be pointed to the right location. However if you didn't calculate for declination cause true north moved you'll be facing a few degrees off.

​

I'm sorry if this explanation is confusing. It's typically best done with a physical compass and map to show what i'm talking about. I tried my best to do it in writing.