Rotary District Designated Funds (DDF) are a portion of a district's contributions to the Annual Fund - SHARE. Donations in this fund are invested for 3 years, after which about half becomes available to the district. DDF can be used to fund district grants, global grants, district projects and more.
Here is a link to more information on Rotary International's website - https://my.rotary.org/en/participate/support-our-foundation/annual-fund