It’s for antenna input although it seems it won’t be integrated onto the board until the firmware support for it has been worked out (it probably costs too much to include just for good measure.)
If it’s going to remain the Skyworks Si2177 tuner, it will be multistandard (B/G, D/K, I, M/N) and the existing ADV7280 video decoder in turn decodes pretty much all flavors of PAL, NTSC, and SECAM. Sound in turn will be mono only (no MTS, A2, or NICAM) as there are no means of decoding other formats, but since no console or home computer did stereo sound over RF as far as I know that won’t be an issue.