That command flushes the DNS resolver cache. The Windows DNS resolver checks the cache before querying a DNS server, which speeds up queries and decreases network traffic.
My ISP member told me it increases the speed for surfing. Is it true?
No, in fact it will do the opposite until the DNS resolver cache is reloaded.
Note that the DNS resolver automatically discards records from the cache after the amount of time specified in the Time to Live (TTL) associated with the record, so manually flushing the cache is not normally necessary or recommended. If you do flush the cache, the system must query DNS servers again for any DNS resource records previously held in the resolver cache. Scott.