I just ordered on of these for myself. I have the Samsung Galaxy S5 (an AMAZING phone). This is one of those things that, after seeing it, you smack yourself in the head and say, “I should have thought of that!”
In short, it’s a button that goes into the headphone jack. It has an app that lets you customize what the button does. For example, press it once to open the camera app and take a picture. Genius!
Best of all, it’s $27.