.30-06 Shoulder Set Back

I have just joined this forum finding it this week while reserching loads for a new caliber for me. I came across an article in the forum asking about the amount of shoulder set back on .30-06 rounds. I had never heard the term and did some more looking. One article stated the Speer manual talked about this but I couldn't find it in mine.

I have been loading .30-06 for my rifle for years. I have always full length resized the brass including lube inside the neck. Probably shot 500 or more of my loads in the rifle and have not had a problem. Very accurate, no chambering problems, no mis-fires, etc. I am about to do some loads for a friend who has the same rifle as me. We have checked the OAL needed for his rifle and it measures the same as mine. I gave him some of mine to test in his rifle but he hasn't had time to shoot them yet.

My questions are: (1) what really does shoulder set back mean. (2)If the cartridges load and fire is there a potential safety problem, (3) Is there something else I need to check or do before I load rounds for a different rifle? If I haven't got all the proper questions on this post, I would appreciate your educating me and helping with my ignorance.