Cycles, also known as septi-cycles, are 175 hour "finish lines" that average the MU from 35 Check Points. The score is counted at the Global level and, as of v1.43.1, the Cell level.
The ongoing tally can be seen under the "Intel" tab of the OPS menu. Previous cycle scores can be viewed by tapping "History" at the bottom left corner. In addition, the top 50 agents from each faction are ranked by their MU control throughout the cycle, which can be seen by tapping "See other agents" under the cell map.
Cycles have no real effect on gameplay, other than acting as a scoreboard. However, in rare cases cell scores at the end of a cycle have been used to determine the outcome of an anomaly, such as during Helios.