A value of true (the default) indicates normal servlet processing, where multiple requests can be processed simultaneously with a single servlet instance, under the assumption that the author synchronized access to instance variables.
A value of false indicates that the servlet should implement SingleThreadModel.
When you say isThreadSafe is true, you are going to take care of multi threads. When isThreadSafe is true there will be on JSP instance serving multiple requests, one thread will be created for each request to acess jsp service method. If isThreadSafe is false, for each request there will be one jsp instance(internally corresponding servlet class for jsp)