Your Feather Radio does not have a built-in antenna. Instead, you have two options for attaching an antenna. For most low cost radio nodes, a wire works great. If you need to put the Feather into an enclosure, soldering in uFL and using a uFL to SMA adapter will let you attach an external antenna

Wire Antenna

A wire antenna, aka "quarter wave whip antenna" is low cost and works very well! You just have to cut the wire down to the right length.

Cut a stranded or solid core wire to the proper length for the module/frequency

433 MHz - 6.5 inches, or 16.5 cm
868 MHz - 3.25 inches or 8.2 cm
915 MHz - 3 inches or 7.8 cm

Strip a mm or two off the end of the wire, tin and solder into the ANT pad on the very right hand edge of the Feather

That's pretty much it, you're done!

uFL Antenna

If you want an external antenna, you need to do a tiny bit more work but its not too difficult.

You'll need to get an SMT uFL connector, these are fairly standard

You'll also need a uFL to SMA adapter (or whatever adapter you need for the antenna you'll be using, SMA is the most common

Of course, you will also need an antenna of some sort, that matches your radio frequency

uFL connectors are rated for 30 connection cycles, but be careful when connecting/disconnecting to not rip the pads off the PCB. Once a uFL/SMA adapter is connected, use strain relief!