Push anywhere on the trackpad to click.
Push anywhere on the trackpad with two fingers, like a cross between scrolling and clicking. Alternatively, hold the "control" key on the keyboard while clicking to get the same result.
This is used when things can be selected, but you want to open them instead, for example to make a picture bigger in Photos, or to open a file from the Finder. This is a faster way to do File>Open.
See: https://help.apple.com/machelp/mac/10.12/index.html?localePath=en.lproj#/mh35853 for more info.
Video from Apple Menu > System Preferences > Trackpad.