Yes, a better explanation of the question would be useful.
Where did you hear this term and in what, uh, context was it used? :)
The only reference I can think of is SQL vs. PL/SQL context. If, for example, you have some PL/SQL code, a PL/SQL engine will be riunning it (you are in PL/SQL context). This engine could be the one on the server or could be a local engine that exists with some Oracle tools (Forms, etc.). In the middle of this PL/SQL code, you have a SQL statement. This statement will definitely go to the server and be run by the SQL engine there. You are now in SQL context. If, inside this SQL statement, you call a PL/SQL function, you will be switching out to PL/SQL context and running the PL/SQL function in a PL/SQL engine each time the function is called (possibly every row if the function is in the SELECT clause). Basically, this 'context-switching' is expensive and should be avoided, all other things being equal. Of course, if you absolutely need the PL/SQL function, then so be it, and you have to deal with the overhead.