In the context of platforms like HDMovie2, "extra quality" typically points to several technical standards:
Streaming (in most jurisdictions) is a grey area, but or torrenting the "Extra Quality" files is direct copyright infringement. Your ISP can track these connections. In countries like Germany, the US, and the UK, fines can range from $500 to thousands of dollars.
The operators know you want extra quality. They use that desire to force you into creating accounts or completing surveys to "unlock the 4K stream." This is 99% a phishing trap.