Headers are like the duct tape of electronics. They're great for connecting things together, soldering to perf-boards, sockets for wires or break-away header, etc. We go through these guys real fast, and thought that given how handy they are, we'd offer them in a pack of five!
Stacking headers are a twist on the standard/common male and female header sets. In this case you have one header that has a socket on one end, and a long tail that gets soldered through a PCB. When soldered into a through-hole pad, the long tails can plug into another board's female headers while the sockets above the PCB allow another board to be plugged in. This means you can stack PCBs, as many as you like, on top of each other. You can even plug the lower board into a solderless breadboard, or plug wires into the top board's sockets.
If you are looking to specifically stack Arduino shields, we have an Arduino stacking header set. For Feathers, check out this Feather stacking kit. For Raspberry Pi and others, check out our other stacking options.
If you want to stack your own boards, or one that doesn't have an existing kit, this pack gives you flexibility to add stack-ability to anything you've got! Each pack contains five 36-pin long female stacking headers. They're even gold plated so they wont oxidize! You can cut these down to different sizes fairly easily. First, count how many positions you want as the final piece, then pull out the next pin using a pair of pliers. Finally, cut through the empty spot with diagonal cutters. You can clean off the half-spot using a file or with the diagonal cutters.
Note: Feathers or other PCBs not included. You're just getting the headers here!