Dear Saint,
You are going to hear a number of explanations about this simple saying of Jesus. Some are going to try and qualify it with extra requirements and religious sounding arguments. Some will dismiss it as referring only to spiritual things. But the answer is simple. He meant exactly what He said! If you pray for something, and believe that you receive, then you will have whatever you ask for.
Mark 11:24 WNT
(24) That is why I tell you, as to whatever you pray and make request for, if you believe that you have received it it shall be yours.
The only qualification is: do you believe?
Some may try and justify themselves, thinking that because they do not receive, it is a slight to the estimation of their faith. Yet no one is accusing anyone of being unspiritual or somehow lacking in faithfulness. They may even attempt to reconstruct the very nature of God into the image of their own unbelief, saying that God does not really want the best for you, and even going so far as to say that God wills for you to be without: without health, without safety, without needs being met. They would rather impugn God as an uncaring, miserly Father than to admit that maybe they need to grow, learn, and believe.
None of this is true. It is always the will of God that you have the things you ask and believe for. I will say it again. It is always the will of God that you prosper, be in health, and be overrun with blessings in every area of you life! I would rather admit my own lack of faith than to portray God as anything less than the Lord that healeth me, the God who is more than enough, my deliverer, savior, and King.
If we do not receive, the answer is simple. We do not believe. There are numerous reasons why people do not believe. Perhaps the thing we are asking for is simply beyond our personal ability to believe. Perhaps the thing we are asking for is outside the parameters of what our spirit can believe for. Your righteous spirit of God cannot believe for evil. You cannot ask for God to murder someone. "Love believes all things," (1 Corin 13) and murder is outside the realm of love, and so therefore cannot be believed for by your righteous spirit. In these and other examples, the will of God is always the same. He always says yes to whatever you can believe for. He said yes to everything on the day that He sent Jesus to the cross for us. He deemed on that day to settle forever the needs of all mankind on every level.
2 Corinthians 1:20 KJV
(20) For all the promises of God in him are yea, and in him Amen, unto the glory of God by us.
When He made Jesus Lord, He made all things His, and there is nothing that He does not have or that He cannot give to us. To say that God says no is to deny the Lordship of Jesus. When we say Jesus is Lord, we are saying that God said yes to everything. He has given us everything. He has nothing left to hold back.
2 Peter 1:3 WNT
(3) seeing that His divine power has given us all things that are needful for life and godliness, through our knowledge of Him who has appealed to us by His own glorious perfections.
Oh saints of God, the devil is waiting to steal this simple truth from you. Do not believe his lies. Accept the plain and simple truth of the goodness, the power, and the love of God. Accept it as a little child. In Jesus Name! Amen!