Syntax for simple text log filtering

Hi guys. What is this search field called and how do I use it? There are what seem like a great many different ways to query logs in cloudwatch, but I can't figure out what syntax is meant to be applicable to this simple one-line search tool you see regularly during object inspection. I haven't been able to work out how its special characters / pattern matching work through trial and error. My logs are all just unformatted plain text in this case.