What does the colon (':') mean in SQL syntax?

Possible duplicate:
What does the colon ":" in the SQL query mean?

Simple SQL Question: What does it mean :?

For instance:

SELECT * FROM myTable
WHERE Employee_column = :P_EmplId;

:not quite easy to use google if you don't know what it's called. Even the search here did not help. I am using Oracle 11g if that matters.