Do we have to watch this film in order to understand plot points and characters of Godzilla: King of the Monsters (2019)?
Do you have to? I would say no.
Although I am not seeing the film until tomorrow, only a few of the characters are returning from the prequel, these being Scientists (Dr. which are following and tracking the existence of these 'Titans'. Not offering much to the over arching story of the series so far.
The prequel is self contained and doesn't offer any expanding plot points behind a post credits scene that has small pictures of other 'Titans' as far as I know.
UPDATE: After seeing 'Godzilla King of the Monsters' I can further recommend that you do NOT need to have seen Godzilla (2014), the points above are still relevant