'Navratna' is a category of Indian Public Sector Enterprises (PSEs) identified by the Government of India as "public sector companies that have comparative advantages", giving them greater autonomy to compete in the global market so as to "support [them] in their drive to become global giants".
Few examples of Navratna companies are BPCL, HPCL, BEL, HAL, OIL, PowerGrid, RECL etc. You can find the complete list of these companies here.
Invest in Navratna companies can provide mixed opportunities. Few of these stocks have given amazing returns in the last few years. On the other hand, there are few which hadn't created any wealth for their shareholders.
For example, HPCL has given a return of 428.87% (over 5x) in last year.
On the other hand, Power finance corporation limited merely gave 21.79% returns in the last 5 year.
Overall, it's the 'fundamentals' of the company that matters more than the 'TAG'.
Nevertheless, these companies can be a good investment for the long term, if you invest in the right stock after doing a proper study.
I hope it helps. Happy Investing.