"I tell you the truth." This adds emphasis to what Jesus says next.
# if you have faith even as small as a grain of mustard seed
Jesus compares the size of a mustard seed to the amount of faith needed to do a miracle. A mustard seed is very small, but it grows into a large plant. Jesus means it only takes a small amount of faith to do a great miracle. (See: [[rc://en/ta/man/translate/figs-simile]])