Abacus is a passive generator program which generates +4 PWR at the start of each mission. It can be bought from some Server Terminals for 400 credits. It has no other effect for the remainder of the mission.
Abacus and Abacus 2.0 are considered by many players to be almost worthless. This is because they generate so little PWR per mission compared to other generator programs, and because using one of Incognita's program slots for Abacus or Abacus 2.0 means one fewer slot is available for other programs.
When playing with 0 starting PWR as a custom difficulty setting, it is possible that Abacus or Abacus 2.0 might be useful in order to start generating PWR using Fusion, or Prism's Refraction Chamber augment. However, there are usually better options.