[Stinged]

Just like Vince said, it means it will only update the first row it finds.
I'll give an example using a picture:

Code:

UPDATE table_name SET Level = 500 WHERE Name = 'John' LIMIT 2

This will set the level of the first 2 to 500.

Code:

UPDATE table_name SET Level = 100 WHERE Name = 'John'

This will set the level of all of the rows that have John as name.