A sensor index is a number corresponding to a specific registered sensor. A sensor index can then be used to query data for that sensor from our API. You can examine our Sensor Index article for further information and instructions on how to obtain an index.
Sensor read keys are similar to sensor indexes. They are alphanumeric and are required to obtain the sensor’s data from the API if it’s privately registered. Information on obtaining a sensor’s read key is found in my response to this How do i get my sensor Read Key post.