Pogo pins are little spring-loaded contacts, very handy for making jigs, or making momentary (but electrically solid) contacts. Adafruit use them by the dozen for making programming and testing jigs but they're handy also if say you want to JTAG program a board that you cant solder headers to - just shove these thin contacts into a 0.1" pitch IDC programming cable and press it onto the contacts.
These particular pins are (in Adafruit's opinion) ideal because they are skinny enough to fit into most proto board or perf board - they have a 1.0mm/0.04" diameter body (the gold part) but the spring contact head is 1.5mm (0.06") wide. The uncompressed length is 16.5mm/0.65", compressed is 14.5mm/0.57"
- Diameter (body): 1mm / 0.04"
- Diameter (spring contact head): 1.5mm / 0.06"
- Uncompressed Length: 16.5mm / 0.65"
- Compressed Length: 14.5mm / 0.57"