[Zope] What "Maximum age of a cache entry (seconds)" means in RAM Cache Manager?

I have read the zope book and zope online help about RAM Cache Manager. But all of them didn't say anything about the parameter "What "Maximum age of a cache entry (seconds)" mean. It looks that this parameter like "Cleanup interval (seconds)", which zope book explained as "The lifetime of cached results.". But what this parameter mean? Would somebody give me an explaination? Thank you.