In computer science, something is high level if it provides an overview of a system or structure.
Python, Perl, and Ruby are considered high level programming languages because they abstract away much of the functionality that is close to the machine.
High level documentation speaks in simple terms and explains what a function does, not how it is done. For example, "This function calculates sin(x)" is a high level documentation of a function. "This function calculates the ratio between the measure of the angle opposite to the one in question and the measure of the angle hypotenuse of a right triangle." is an example of low level documentation.
High level diagram refers to a diagram that ignores details. For example, a diagram explaining the client/server architecture of a system that displays an accounting department, sales department, and production department all connected to a main server is a high level diagram. If the same diagram includes the specifications for the type of wiring to use and the number of switches and routers needed, it is low level.