InfluxDB is a time series database that has been designed that way, as opposed to a relational (or other) database that supports time series. It is the leading pure-play product in this space.
There are two particularly prominent use cases for InfluxDB. The first is IoT, where, it is used to support the processing of large quantities of sensor data in real-time. The second is in operations (including DevOps) and data centre environments, where you wish to monitor and analyse your infrastructure and/or business processes, also in real-time, and then take actions based on the results. The bulk of the company’s users are in the latter category though IoT use cases are increasing.