Lua...stuff...helped

OK im learning a lot of lua(non-principia) and I know that its handy to have a script that is constantly checking something like a particular variable, so you can use it with changing variables(like an input in principia, say from an object finder). How can I do this? I see lots of lua scripts is principia that seem to be running the script indefinitely.

For example in this level, the script is run only once at the beginning of the level. Else the message would constantly change. I want to know how to run script 'indefinitely'. If you can help I would appreciate it very much! Sry for the text block. Just wanted to be clear so I didn't sound like "LUA HELP HOW I DO _________ ???"