Doom Eternal is doing regular in-game events, dubbed Series. I don't like the name and it makes the whole thing confusing, but suffice to say Series 2 is here and brings with it new weapon and character skins, icons, and nameplates. Basically you play the game and unlocked timed exclusive customization content. This time around we've got Camo for your Slayer suit, Camo for your guns, and even a coffee stand to let out your inner demon hipster.
Full details on Series 2 is available on the Bethesda Slayersclub site. That's where you also found more info about Series in general. I'm still working my way through the Doom reboot from 2016, which is incredible, before loading into Doom Eternal, and even though that means I'll be missing out on these timed cosmetics, just joining the Slayers club will net me loot and I don't really get the FOMO when it comes to cosmetics anyway.