accompanying note; cover letter; note enclosed with something
A short letter or note sent along with a gift, document, or package, explaining the contents or conveying a message.
I sent the gift with an accompanying note.
履歴書に添え状を同封してください。
Please enclose a cover letter with your resume.
Compound of 添え (soe, 'accompanying') + 状 (jō, 'letter'). The exact historical derivation is uncertain, but the word follows a common pattern for naming types of letters.