Middle level, the "Height" of a language represents how abstracted the language is from what the computer actually does. For example, a high level language might say something like:
SELECT * FROM PEOPLE WHERE AGE > 50
These (SQL) instructions cause a massive host of different things to happen on the computer, it essentially does a search through a database and builds a set of results for whatever program exectuted the SQL. The point is that the language doesn't show any of that, you just say "Get me this" and leave it up to the system to figure out how to do it.
In a lower level language an equivalent might be:
for (int i=0; i
{
Person *p=&people[i];
if (p->Age>50)
{
AddToResultSet(p);
}
}
As you can (hopefully ;) ) see, the psuedo C above goes through an array of data structures checking the age field in the structures and adding the data from those structures that meet the given condition (age is over 50) to a result set somehow. This code, when executed, represents fairly literally what the computer will be doing and how it does it, there is very little abstraction.
I hope that makes sense, it turned out to be harder to explain than I thought it would...