Mods for PUBG are extensions on the client or server side of the game that make significant changes to the gameplay. At the moment, the developers have not yet added an open ability to create mods for Playerunknown's Battlegrounds, however, this will soon be fixed.
In an interview to the question "what is the place of modding in the gaming industry, should developers support or at least not interfere with this process?" Brendan Green replied as follows:
Dota, CS, Battlegrounds, DayZ all these games appeared as mods from the very beginning. In creating mods, the freedom of expression of the author's intention is important, there should be no formal restrictions like instructions from above. This freedom is very important for a game designer. And in this sense, personally, I believe that any game should be a mod. Each developer should be able to create their own version of the game.
This position gives a chance that an official SDK or some other possibility of creating custom content will be released this year.