It means
SL has a new, much more efficient scripting engine that will also eventually allow a lot more programming options in scripts. However, it runs along side the current one, it doesn't replace it. Only new items created specifically with Mono scripts in them will use the new engine - existing scripts in items will continue to use the existing one, so you have nothing to worry about. You can take your existing scripts in your objects and recompile them in Mono, but you don't have to. It will mostly provide benefits for complex scripts, not for the simple scripts you probably have in furniture and other items. You will be able to continue to use your old scripts in new items you develop as well. Ideally you should switch them over eventually, which will just mean recompiling the script in the object, not rewriting it.
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"Moving on...Police warn that the Sudoku Killer will kill either 1, 4, or 9 victims next" - the Onion