OBD (On Board Diagnostics) describes data and information that is available from a connector inside your car - if it is built after 2004, or if you are lucky as early as 2000. It contains not only error messages, but live data from the onboard computer. This includes for example the engine load, oil temperature, fuel usage, etc. - you can find a full list here, but keep in mind that your car may not provide all the information.
The idea of the OBD Logger is to find a way or an already existing solution to capture this data. After that it can be determined what information is available and also what other information can be derived from that - call it data analysis if you like.