Cinematic Mode is a mode where almost all the GUI is not present, including your ammo counter, health, etc. It was added in the Flare Domination update, which gives players the unique experience of realism. However, some aspects of the normal HUD remain, including spotting, kills, rewards, and ranking up. It also adds the challenge of telling apart friend from foe, as there is no HUD to tell you. There are also no crosshairs, which unless you have a laser sight, or frequently ADS, will be a challenge. It also forces players to guess how much is in the magazine, leading to more conservative gameplay.
Normal HUD for reference ->
Cinematic Mode ->