HD: HD stands for Henry Draper Catalogue, which is a stellar catalog containing information about thousands of stars. It was compiled by Annie Jump Cannon and her colleagues in the late 19th and early 20th centuries.
The number represents the specific star in the Henry Draper Catalogue. In this case, HD 189733 refers to the star around which the exoplanet was discovered. The number of the star has no meaning outside of that it was the 189733rd catalogued one in HD's catalogue
The letter "b" is used to designate the first known planet orbiting the star. (a is for stars) If additional planets were discovered in the same system, they would be labeled with subsequent letters in alphabetical order (e.g., HD 189733 c, HD 189733 d, and so on).

Therefore, HD 189733 b signifies the first planet discovered orbiting the star HD 189733. This naming convention allows astronomers and researchers to identify and differentiate between different planets and their host stars in a systematic manner.

TOI 849 b is TESS Object of Interest's 849th discovery of interest. The numbers are essentially arbitrary outside of that numbers before them have already been taken.