It says in this book that I am reading that it is good to have a remote shutter release, but I have no clue what it is. I have tried searching the web but nothing answered my question.
Adding to @romeoninov's answer:
Modern cameras can also work "tethered": some link (USB cable or wireless with Bluetooth/Wifi) connects them to a device (PC, tablet or smartphone) that runs a dedicated application (usually available for at least Android, iOS and Windows). With this you can:
- trigger the shot on the camera (like with a remote shutter release)
and optionally (depends on camera and application):
- view the resulting picture, and sometimes download it directly to your device
- preview the picture that is going to be taken (like LiveView on the camera rear screen, but on the remote device)
- change the camera settings (ISO, speed, aperture)
- adjust focus
- adjust zoom factor (requires a lens with electric zoom, rare on DSLR, but possible on "bridge" cameras...)
The wireless part is especially useful if you can't be close to the camera (taking self-portraits, for instance)
This can replace may uses of a traditional remote release.
This said, I use both, plugging in a traditional release is faster than initiating the connection with the smartphone.
As it name said this is something which help you remotely to "press" the shutter release. It can be mechanical (usually for old film cameras) or electric/electronic (for contemporary cameras).
What this can help you? When you press shutter release you add vibrations to the camera which can introduce motion blur to your image (which in most of the cases is a problem). When use remove shutter release you "deattach" those vibrations from the camera.
One usual use case is: camera set on tripod, remote shutter release to make camera as much stable as possible during take of photo. Or another use case is to use BULB mode and expose longer than 30 seconds (usually maximum exposure you can get on Av, Tv, M modes).
Of course there are other ways to avoid those vibrations (take photo with 2,5,10 sec timer for example). But remote shutter release can offer (sometime) additional options like timelapse or shooting objects with distant camera from shelter.
As mentioned in comment it is good to use mirror lock up option additionally to above methods to avoid vibrations introduced by mirror movement (when we talk about DSLR cameras)