It's similar to the quadrants in the cartesian axis, you can consider it analogous to the 2D quadrants. Like how you'd use quadrants to refer where a point is on the xy axis, you'd use octants to describe where a point is on the xyz axis.
I doubt you'll be tested on knowing "octants" in an exam Q but perhaps the marker might assume you know it and use the terminology to describe what specific axes does a point in the 3D space lie in.