What makes a pseudorandom generator, a high quality one?

Question

Reading this answer to this SO question: Why do we not combine random number generators?, it talks about

very high-quality PRNG (Pseudo Random Number Generator)

so it makes me wonder what constitutes a high-quality PRNG, I assume you can sum it up as it being "more random", but

• Question1: Which qualities of a PRNG are used to describe how 'random' or 'good' it is?

• Question2: If you have a 'bad quality' PRNG, is there a way to make it better quality?