Acute:Chronic Training Load (ACTR) is the ratio between training load over the previous 7 days (acute load) the average training load for the preceeding three 7 day periods (chronic load). Training load is measured using heart rate and duration data. This approach is a refinement of the 10% rule which runners have been using for years and which suggests that you should not increase training volume by more than 10% week on week.
Research suggests that when the ACTR gets higher than 1.3 the chance of injury begins to significantly increase. When training load drops significantly then detraining can occur.