The Mustard plant grows to a remarkable size in comparison to the original mustard seed and Jesus referred the seed to the principles of the Kingdom of God as a living thing that grows within us to produce remarkable things
In other words, it only takes a mustard seed amount of faith (confidence) in the principles of the Kingdom of God to remove huge barriers such as mountains. Jesus said if you have a mustard seed amount of faith in the Word of God, nothing would be impossible to you.
Jesus also says believers could speak to the mountain and it shall be removed. Now, will you believe with me? Because I believe Jesus, I can say to my mountain (or any barriers) “move here to there”and nothing will be impossible for me. By faith I say I believe, will you too?
So Jesus said to them, “Because of your unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you.