It is a “Fast Mission” opportunity. These are missions that address an opportunity in the near future that would possibly be missed with the normal mission development timeline.
Historically, ESA missions are classified as Large (L), Medium (M) or Small (S). The distinction is not on size of the payload, but on the technology development required for the mission, and consequently the number of years required to get from idea to launch:
[...] for flagship large (L-class) missions [...], it can be up to seven or eight years, because these missions generally require cutting-edge technologies and demanding pre-developments to enable the mission adoption. For medium-sized (M-class) missions, the preparation phase is about four to five years with (limited) technology developments. For fast-track small (S-class) missions, this phase is less than two years with no technology developments.
In 2018, the F-class was announced in addition to the existing classes. These missions are small (under 1000 kg including fuel), cheap (under 150 M€) spacecraft that can piggyback on one of the upcoming M-class mission launchers:
This Call for a Fast mission aims at defining a mission of modest size (wet mass less than 1000 kg) to be launched towards the Sun-Earth L2 Lagrange point as a co-passenger to the ARIEL M mission, or possibly the PLATO M mission.