What did Tom and Huck do to take an oath in order not to reveal what they saw ? Why did they do it

Not knowing that Injun Joe is attempting to frame Muff Potter for the murder, the boys decide to not tell a soul about what they had seen for fear that Injun Joe would seek revenge upon them as well. They sign a contract to keep their secret "mum" (an image of the contract in Tom's handwriting is placed within the text) and sign their initials in blood after pricking their fingers with needles.