An activity bar is an indicator which shows a player's level of activity during various minigames. It was first introduced to Soul Wars, to help reduce the amount of freeloaders (people standing in the game without doing anything to help, solely to get free Zeal).
After proving its efficiency in Soul Wars, it was later applied to both Fishing Trawler and The Great Orb Project to help with the same problem. 2 to 3 minutes of inactivity will result in the freeloading player being kicked out of the game and receiving no reward. However, many claim that it is ineffective, as freeloaders often do nothing until their activity bar is near empty, then help to get it back up, and afterwards they start to do nothing again. However, the bar sometimes unevenly drops, and so freeloaders may not have enough time to get their bar back up in time. The activity bar level can be raised by moving orbs (Great Orb Project), patching up the trawler (Fishing Trawler) or by engaging in combat or offering soul fragments to the Soul obelisk (Soul Wars). Depending on which of these three minigames you are playing there may be other possibilities.
Sometimes, while playing Fishing Trawler, if the boat floods enough, it is possible to keep your activity bar up solely by clicking your bailing bucket. This has lead to some players abusing this opportunity by using an auto-clicker, which is against the rules.
The activity bar is often an annoyance to some players in Soul Wars, as they sometimes won't be able to any action before they are kicked out of game due to serious lag.
The activity bar is found in these minigames: