It is used to say that a low-level language has more access to CPU-Specific operations. You know every program is made of little instructions in machine code.
Assembly is the lowest-level language at this time i'd say.
Then there are mid-level languages. I'd put C and C++ in here, because I don't find them really low-level, but you can specify inline assembly code.
And then the high-level languages. These are the easiest (but most time least performing) languages. They are made to make things easier to programmers.
I guess VB is a high-level language?
Low versus high also relates to the level of abstraction the language provides. A low level maps more closely to the computer's instruction set and architecture.
A high level languages works with more powerful and/or human friendly constructs that must be converted to something the computer understands using a compiler or interpreter.
As assembly language is a mnemonic form of the machine instruction set, it's as low as you can get (unless you code the op codes in directly!).
I think that C++ is a bit unusual in that it provides quite a high level of abstration (classes, templates, etc) while also letting you get at the lower level stuff (Wikipedia.org refers to it as an intermediary level language).
Some high-level prgramming languages provide a powerful abstraction without being anything like human language, like APL and LISP. But they are easier to read than assembly language.