Learn Swedish: Lantstället vore mycket avkopplande. - The country house would be very relaxing.

Vore is a special, single-word form meaning 'would be' (the past subjunctive of the verb vara). You can absolutely use skulle vara here, and it means exactly the same thing. However, vore is very common in hypothetical statements like this and sounds very natural to native speakers.

How is the word Lantstället put together?

It is a compound word made of lant- (referring to the country or rural life) and ställe (place). The -t at the very end is the definite suffix, making it 'the country house' rather than just 'a country house'.

Why does avkopplande end in -ande, and why is av at the beginning?

This is the present participle (the '-ing' form) of the phrasal verb koppla av (to relax). When you turn a phrasal verb into this kind of adjective, the particle (here, av) moves to the front to form a single word. The -nde ending works just like '-ing' in English, giving us 'relaxing'.

Could I say jätteavkopplande instead of mycket avkopplande?

Yes, you definitely could! Mycket avkopplande is standard and works well in both writing and speech. Using the prefix jätte- (like jätteavkopplande) means the exact same thing but has a slightly more casual, conversational tone.