rooted and built up in Him and established in the faith, as you have been taught, abounding in it with thanksgiving.
Beware lest anyone cheat you through philosophy and empty deceit, according to the tradition of men, according to the basic principles of the world, and not according to Christ.
Colossians 2:6-8 (NKJV)
Trees need to have good roots to grow and develop; a tree without a developed root system could easily be uprooted. A tree that is deeply rooted has more stability than a tree without a good root system. If some storm would come, or something would run into the tree with enough force, the tree without the deep roots could be easily uprooted. No tree could survive if it was planted in one place for a short time and then uprooted and put into a different place again and again. Trees need strong, developed root systems to grow.
In Colossians 2, Paul talks about how those who have received Christ need to walk (to live) in Him, and be rooted and built up in Christ, established in the faith. We are planted and rooted in Christ at salvation, but there are things that could uproot us and lead us away from where we are supposed to be, which is what Paul was talking about. Once we are planted in Christ, we then have to grow and develop in our knowledge of God and His Word to develop deep roots in Christ and be built up and established.
Paul warned us to be on guard so that no one would "cheat" us, which means to be carried away, through wrong doctrine. Paul is saying that through wrong teaching we could be taken away from where we should be, we could be uprooted, instead of being deeply rooted in Christ. By spending time with God in His Word and prayer, and spending time with other Christians, we can develop deep roots in God and our life in Christ. We need to be rooted and built up in Christ and not let ourselves be uprooted and led astray by anything or anyone.