The context of these verses:
The day before when Jesus was hungry, He had gone to a fig tree but found nothing but leaves. So Jesus spoke to the tree saying, “May no one ever eat fruit from you again.”
The next day as the disciples passed, they saw the fig tree withered from the roots … Peter remembered Jesus’ words and called attention to the tree.
“Have faith in God. I tell you the truth, if anyone says to this mountain, ‘Go, throw yourself into the sea,’ and does not doubt in his heart but believes that what he says will happen, it will be done for him. Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours. And when you stand praying, if you hold anything against anyone, forgive him, so that your Father in heaven may forgive you your sins.