(SFI) F10.7 - Solar Flux Index: The amount of flux (radio noise) emitted at 2800 MHz (10.7 cm) wavelength;
(SN) SSNe - Effective sunspot number derived from ionospheric measurements;
(X-Ray) - Solar x-ray flux over the band 1-8 Angstroms;
(A-Ind) Ap - Daily global index of geomagnetic activity (preliminary);
(K-Ind) Kp - 3-hour global index of geomagnetic activity (preliminary);
(304A) - UV emissions at 304 angstroms (UltraViolet), correlated to Solar Flux Index;
(Ptn Flx) - Proton Flux: Strong enhancement in energetic proton flux (typically >10 MeV), commonly from a shock caused by an energetic solar eruptio;
(Elc Flx) - Electron Flux: The electron flux measured by the GOES satellites indicates the intensity of the outer electron radiation belt at geostationary orbit;
(Aur Act) Qe - Effective Q index (auroral activity index);
For an outstanding explanation of all this, watch this short video: