we are writing <%@ page iserrorpage="true"> what it actually .I mean it is Error object or else other

isErrorPage is a attribute of page directives and which is used to check whether particular jsp program having error or not.

Actully this is an attribute of this page, and this particular page will be displayed when the other page( say page 2) got some error, provided, where the errorpage attribute  has to de set to this page. put it in a nut shell, this shows that is going to act as an error indication to some other page

If in MyErrorPage.jsp

<%@ page isErroPage="true" %>

is written it doesn't mean this is an Error Object.
If this page is included as error page in any other jsp (suppose MyPage.jsp) using

<% @ page errorPage="MyErrorpage.jsp" %>

then if any Exception is occured in MyPage.jsp then equivelent Exception Object will be created and request will be forwarded to the MyErrorPage.jsp this will generate response so here you can handle the Exception.

iserrorpage is attribute of page, While executing the page it throws any errors then it returns Exception Object