What does an RF coupler actually do?
Here's a couple of ways of drawing a directional coupler in a block diagram.



(image source: Wikipedia)

Most of the signal incoming at Port 1 (P1) will pass through to P2, since they're connected by a transmission line. And most of the signal incoming at P2 will pass through to P1. 

"Coupled" just means (partially) connected. 

The key idea of a coupler is that a fraction of the signal coming in on Port 1 (P1) will be "coupled to" the output at P3. Meaning the signal will be output from P3. What makes it a directional coupler is that (ideally) none of the signal entering P2 will appear at P3.

Similarly, the output at P4 will be coupled from P2, but not from P1. Often, the P4 output is terminated internal to the coupler device, so you don't have access to it, and then you'd use the second style of symbol from the image.