It means that anything over that limit will be rejected with a 500-Internal Server Error code.
To change what it does when it hits that error the
SecResponseBodyLimitAction directive affects that.
By default, ModSecurity will reject a response body that is longer than specified. Some web sites, however, will produce very long responses, making it difficult to come up with a reasonable limit.
Reject causes the 500 error, where
ProcessPartial allows even long responses to be allowed, but only the first part of long responses will be processed against mod_security.