Ambient DMing includes all IC DM activity which is not covered by events. Examples include idle banter with or between NPCs, redecorating a room for a birthday or adding more snow in wintertime, providing additional descriptions or spawning a crystal item for a PC who breaks off a chunk of their surroundings. The goal is to surprise and amuse players, to encourage them to interact with their environment and to reinforce the setting of our module by bringing it to life. Some light events, like those run mostly by the players themselves with a splash of DM support or ones where an influential PC is approached by an NPC about business could be considered ambient as well.
It is always good to use a light touch with this form of DMing. If players are keeping eachother entertained, there's no need to derail them. NPCs might respond to their conversation or you could make a minor tweak to the environment but overall, let them play. If on the other hand they're struggling for a topic or sitting all alone, you may want to take a more active role and seek out the interaction. Stage NPC banter, invite a PC to a meeting with an official, send someone a letter or outright ask a player if they'd like to discuss anything with an NPC. Your actions can validate RP and help a PC gain new purpose.
It can be difficult for players to tell whether your intervention is innocent background entertainment or an attempt to draw them into an event. Sometimes events grow naturally and it helps to be ready for the opportunity, but if you find yourself with too much attention you may want to clarify your intentions through tells.
You have the authority to control any NPC and to invent new ones too. You may find yourself hesitant to hijack another DM's toy because you wouldn't play the character in quite the same way. This is healthy. Just be aware that you do have the option if the situation calls for it. However, because we're sharing, it's important that you use the forums to record NPC mannerisms (such as speech), motivations, background details and attitudes towards PCs. If you've crafted a custom NPC you may also want to record their appearance. There's no need to write a long and elaborate history, just capture the essence.