First, I'm a bit shocked that you're in VEX and asking what an engineering notebook is. It's a notebook that you and your team log their work on the robot in as you design, build, program, compete with, modify it, etc. It's basically a diary about the robot. It's a part of learning and applying the engineering/design process and teams are very strongly encouraged to keep one. Technically, you're not required to have one for competitions, but it's pretty much a must-have since it lets the judges get an insight into the design process behind your robot, and it is used in consideration for the Design Award and indirectly the Excellence Award, which can get you qualified for higher-level tournaments.
If you haven't kept one, I suggest you get started on it. VEX sells them on their store for $10 per notebook, and if your team has access to a competition kit bought from the store, there should be engineering notebooks that came with it. A rule of thumb for a good engineering notebook is, in the words of my former STEM teacher, where if an external observer can get their hands on the notebook and read it, they can reproduce the robot faithfully. Essentially, whatever you and your team do to the robot should be documented in detail in the engineering notebook.