For little children this phrase reminds them to make sure they haven't switched the letter "p" with the letter "q" and/or vice versa.
For more experienced writers in the English language, it helps us remember to check our work before we turn it in; it helps us prepare for the grade that we will receive.
However, the letter "p" plays a much more prominent role in our lives than you might, at first, suspect. Check it out!
In life, we all makes choices at one point or another that cause feelings of guilt or sorrow. However, through the Atonement of the Savior, Jesus Christ, those feelings of guilt and sorrow can be replaced with peace and joy. We can repair the damage we have caused to both ourselves and others as we sincerely repent of the poor choices we make in life.
Having said this, how much greater is it to "prepare and prevent, than it is to repair and repent?" (notice the importance of the "p")
If we can learn this simple principle of preparing to prevent, we can reduce the amount of grief and pain that we experience in this life and receive a more fullness of joy while in this life!
see Alma 59 for full story).
It is the same way with combating the forces of evil in life.
We too can prepare our minds and hearts to always live a life that emulates our Savior's. We can resolve to keep standards that will protect us from the dangers of this world and in turn keep us both physically and spiritually clean. Thus, we avoid the painful process of overcoming addicting substances and avoiding the painful process of sincere repentance which leads to forgiveness and peace of mind.
I know that preparation will greatly bless one in all aspects of their life. I know that the Book of Mormon is a powerful tool in preparing our minds and our hearts to always follow the example of the Savior.
It is another testament of Jesus Christ and has been prepared for our day and time to help keep us on the "strait and narrow" path that leads to eternal life.