It was vital for Jesus to reestablish prayer in the realm of faith. He had made it absolutely clear that vain repetition of words would avail nothing. Prayer had to be brought out of the realm of self-effort to the realm of the Spirit. Thus He said to the astonished disciples: “Whatever you ask in prayer, believe that you have received it, and it will be yours.”
The use of the word “whatever” means that prayer is no longer to be judged according to the tree of knowledge to good and evil, including its subtle branch: “Is my prayer for others?” It is God who is at work in you to both will and do in accordance with His good pleasure, Paul bluntly wrote. Understood in this context of freedom prayer was to be reinstated as a spontaneous outflow of the Spirit in the believer - whether in intelligible words or not.
And finally, believe that you have received it is another way of saying that the thing is done by God, and that there is nothing you can do to aid Him in the outworking of the manifestation - simply because He is the One who is God. Vain repetition of words or any effort towards assisting God would add nothing to the already finished result. Only believe!
One last thing: When Jesus educates the disciples on marriage and divorce in Mark 10 He says that the two, husband and wife, are one flesh. This is a shadow of our true and only marriage with Christ. Since husband and wife are one flesh, Christ and those who are joined to Him are one spirit/Spirit. Thus, if properly understood, Jesus’ words on prayer in Mark 11:24 make full circle back to Him, meaning that prayer is an agreement between bride and bridegroom.