This is a letter by Tokugawa Mitsukuni, the feudal lord of Mito domain, present-day Ibaraki Prefecture.
Three lines written in large characters are the main text of this letter, which is addressed to a man named Watanabe Wakasa-no-kami. The text reads: "I was delighted that you could visit yesterday. It is a shame we could not meet. I wish to express my gratitude."
Written between the lines of the main text is a two-line addition. It is in smaller characters and written lower on the page. It asks the recipient to pass a sword accompanying the letter to a certain swordsmith. The letter has a somewhat business-like air. In modern terms, it would be like sending a thank-you e-mail and then writing 'By the way, please take care of the matter we discussed' at the bottom.
On the lower left is a figure that resembles a circle next to a triangle. This is an original mark used by Mitsukuni as a signature. Signature marks like this originated in China and were widely used in Japan up until the Edo period, which lasted from 1603 to 1868.