what does it mean by "thread-safe procedure"

I was going thru the help on collection and I came across the following line
"Enumerating through a collection is not a thread-safe procedure"

what is thread-safe procedure..?

thanks

Thread-safe means that the process will work correctly in a multithreaded application.

The reason that iterating through a collection is noth thread safe is that it doesn't lock the collection so other threads can alter the collection while you are trawling through it (with hilarious consequences...not)