A detailed explanation is given on oozie data triggered coordinator job with example.
Coordinator runs periodically from the start time until the end time. Beginning at start time, the coordinator job checks if input data is available. When the input data becomes available, a workflow is started to process the input data which on completion produces the required output data. This process is repeated at every tick of frequency until the end time of coordinator.
The above snippet in coordinator.xml for input dataset signals the presence of input data. That means coordinator action will be in WAITING state till _SUCCESS file is present in the given input directory. Once it is present, workflow will start execution.