Statement of Use and Declaration of Use are documents that US trademark owners must submit to the USPTO. Here's the difference between the two:
You have to submit the Statement of Use only if you applied for a US trademark on the so-called intent-to-use basis, meaning that your trademark wasn't commercially used at the time of applying. Statement of Use is submitted after the application passes the examination and the USPTO sends the applicant a Notice of Allowance.
In an SoU, the applicant declares that the trademark is now commercially used in the US market. The submission of SoU also has to include a so-called specimen - evidence of the trademark being used to label goods/services listed in the application (such as clothing tags, website screenshots, etc.)
Declaration of Use is submitted between Year 5 and Year 6, no matter the type of application. It declares that the registered trademark is still being used in commerce and is available to US customers. Its submission is a mandatory requirement to keep the trademark from cancellation.