When you have an adjective without a noun, the 'Tha seo' acts as 'This...thing' to have a placeholder for the noun. The same way 'This is a book' makes sense, but 'This is a useful' doesn't. So you add 'This is a useful thing' in english to make a grammatically correct sentence.
In english you could also drop the 'a' to be 'This is useful' which is the more natural way to say it in English but thinking about as adding 'thing' makes it easier to see the equivalency.
So Seo leabhar = This is a book Seo leabhar feumail = This is a useful book Seo feumail = This is a useful (which doesn't make sense so we have:) Tha seo feumail = This is a useful thing/This is useful