Foundation is often used to refer to the bottom part of a building that is meant to stabilize the structure. The term can also be used in association with everyday life and what we hold on to in our understandings and beliefs about our daily existence. The whole point of a foundation is to have something that keeps you grounded even when life gets tough and we experience turmoil.
Francis A. Schaeffer wrote many books on the existence and evidence of God in our lives through the use of philosophy, theology, and apologetics. In his book The God Who is There, he analyzes the shift of humanity to post-modern thought. Post-modern understanding is that there is no right or wrong and that all is feasible and allowable within the human context. There are many offshoots of post-modernism, the main idea is that there is absolutely no way to know anything for sure and whatever works for you is what you should believe.
Schaeffer coined the term "line of despair". The line of despair separates those who believe in absolutes and those who believe that there is no wrong way of life. Despair sets in when the people below the line find out that once all things are possible and feasible the meaning of life is no longer an attainable understanding. Many philosophers and intellectuals throughout history have found despair and sorrow at the end of their lives once they found out that all is for naught when you do not hold to any belief system.
The glory and hope of the Christian is that we do not fall bellow the line of despair. We are high above it because of our faith and foundation that is found in the work of Christ on the cross. Jesus wants us to hold fast to Him and the promises He has spoken throughout the Bible. We have an all powerful savior who is the Alpha and Omega, the beginning and end. I Peter 2:6 says "For it stands in scripture: "Behold, I am laying in Zion a stone, a cornerstone chosen and precious, and whoever believes in Him will not be put to shame."" The Lord is our cornerstone and our firm foundation. Believe and hold fast to His work and teaching and He will be with you.