Valve stem seals

They are rubber seals around the, ... Uhm, ... Valve stems. Normally you have to remove the cylinder head to use a valve spring compressor to remove the springs to gain access to the seals. The typical symptom is oil will run into the cylinders after the engine is stopped, and will burn off when the engine is started. You'll see blue smoke from the tail pipe for the first minute of engine operation. If you see smoke for a longer period of time, usually the valve guides are also worn allowing a lot more oil to run in. New seals won't be the permanent fix. That requires a valve job. New seals and / or a valve job are ideally best left for when the cylinder head must be removed for a new head gasket.

Some engines can have the valve seals replaced without removing the head by following special procedures and using special tools. That involves forcing in compressed air into the cylinder through the spark plug hole to hold the valve closed while the spring is removed from on top. That saves a lot of time and expense but is often not possible on many import engines. This works really well on older domestic engines.

The valve stem seals seal the exhaust port from leaking exhaust gasses when on exhaust stroke and intake charge on intake stroke.
They are pressed into the head with the valve then pushed through.
The valve is then held via springs and retainers.
To replace them, generally it is best to remove the head as you will have to retain the valve without dropping it into the cylinder. Also the seals are tight and sometimes damage can occur trying to rove/replace them with removing valve.
To retain the valve some people use a compression test fitting on an air compressor and charge the cylinder to hold the valve. Other makes a, "Hook" type tool to hold it.
Then picking the seals out is tough and usually requires and dental pick type of tool. If the valve get scratched, it won't seal.
It is best to have them out so you can clean and prep surfaces so the seals last longer.
I hope this information helps.