Lucy,
It’s called “dual sound recording” – what you are doing is attaching your lavalier mic or shotgun mic to the Tascam recorder which is obviously not part of the camera. The Tascam has a headphone jack so that you can monitor your sound while recording which is really a must.
Your camera is still recording its own audio but it it will be very low quality, however for the next step it’s good enough usually.
The last step is that in editing you are going to synchronize or “sync” the audio recording on the Tascam to the video clip with the poor audio that your DSLR recorded. Once they are in perfectly in sync you will then turn off the camera recorded sound leaving you with just the high quality sound track in your editing program.
Some people go the extra step and attach a higher quality small microphone to their camera just so that they have better sound being recorded by the camera – it helps with syncing and in some cases (like if you forget to turn on the audio recorder while filming) at least you have some “OK-ish” sound to work with.
Steve Crow