A fly tying bobbin is used to hold a spool of thread used for fly tying and distribute the thread at an adequate speed and tension. There are many types of bobbin's available but most nowadays have a ceramic end to prevent chaffing of the thread. The bobbin shown has foam where the the thread is inserted into the bobbin and requires a bobbin threader to get the thread out of the tip.
A Bobbin Threader is used to pull the thread through the "pipe" on a fly tying bobbin. It's basically a piece of looped wire with a handle or grip fixed onto one end of it.
Just inset the bobbin threader into the ceramic tip and push it down until the wire loop comes through the bottom end of the "pipe", strip about 6 inches of thread off the bobbin and put the thread end through the wire loop of the bobbin threader and pull the thread tight.
Pull the handle on the bobbin threader and remove it from the "pipe". The thread should now have come out too and hey presto , you should have the fly tying bobbin threaded.
TIP: Another method of threading a bobbin holder is to insert the thread end into the bottom of the pipe, then apply suction with your mouth to the top end of the pipe and usually the thread is sucked through. Many say this is un-hygenic but its up to the tyer to decide.