The Page directive of JSP has an attribute isThreadSafe which basically tells the container about the thread-safe behavior of the JSP so that the container can handle the requests to JSP accordingly.
It can have two possible values - True or False. By default, isThreadSafe is True.
When isThreadSafe = true, it tells the container that the current JSP is thread-safe, meaning, it can handle web requests in a multi-threaded way. Each web request is a handled by a new thread of the JSP Servlet. Hence, caution should be taken if you are handling shared resources within your JSP. You may have to use synchronized block to manage shared resources when isThreadSafe is set to true.
When isThreadSafe = false, the container then only handles one request at a time as the JSP is specified to be not thread safe. This option should be used with consideration as it might severly impact the performance since only one request can be catered at one time.