Most people construe this passage to mean that they can ask God for anything.
The previous verse will bring this verse into more clarity:
"And Jesus answering saith unto them, Have faith in God." (Mark 11: 22)
The verse would better read "Have the faith of God."
How do we have God's faith? By receiving Jesus as our Lord and Savior:
"20I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me. 21I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain." (Galatians 2: 20-21)
We are no longer called to live our lives in ourselves, but rather Christ lives in us and works in us, both to will and to do for His good pleasure (Philippians 2: 12-13)
In fact, because of Jesus' death on the Cross, God is able to place His laws in our hearts and minds, so that He leads us from within by His Peace:
"10For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people:
11And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest.
12For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more." (Hebrews 8: 10-12)
We have peace with God, and His love is shed abroad in our hearts by the Holy Spirit (Romans 5: 1, 8), and this same Holy Spirit grants us peace as to what we should or should not do:
"And let the peace of Christ rule in your hearts, to the which also ye were called in one body; and be ye thankful." (Colossians 3: 15, ASV)
In this verse, "rule" means "preside like a judge", in that whatever moves we make which take away peace in our lives, then we do not do those things.
This same peace works in us and motivates us to say and do whatever God wants us to. And we can trust that He will give us all things through His Son (Romans 8: 31-32), that we will be able to do all things that He directs us to do (Philippians 4: 13), and that he will supply our needs (Philippians 4: 19)
More Importantly, He gives us beyond what we can ask or think because of Christ (Ephesians 3: 20)