Valve guide seals?

[Shotgun28]

What exactly does worn valve guides mean? Can they be replaced, or does that mean you need to get a new short block, I am confused. I would appreciate any guidance on this issue...Thank you in advance, as far as I know I do not have this problem, but have heard others mention it.

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91 LX Convertible Gray on Red B&M Shift improver kit,ASP red alum. underdrive pulleys, Richmond 3.73 gears, 1 5/8 Flowtech long tube Headers and off road h-pipe, Flowmaster 2-chamber cat-back with S.S exhaust, MSD 6A Ignition with coil,Cap and rotor, Taylor Pro wires, K&N 12" conical filter system, Jet stage 2 performance chip and just purchased some GT40P heads, looking at exhaust, Lentech Street Terminator plus on it's way...