what some version ios is tethered and other untethered?

what is
ios versions are tethered and others is untethered?? what is the reason??

Tethered jailbreaks are easier to produce than untethered ones.   Therefore the existence of an untethered jailbreak depends on whether those who produce the jailbreaks are prepared to put the additional effort in to make them.  For many firmware releases Apple releases an update before the untethered jailbreak gets produced, so in that case it never gets done.