For more information, please see your Graphical Guidance book where an entire section is devoted to MOS for many of the models. In this case, MDW is the id for Midway airport. Day is simply the day. Hour is the hour that the forecast is valid. Knowing the day which is above it, you will then know when each parameter underneath it is valid for.
TEMP is the temperature for that time.
CLDS represents clouds. Just add a zero to the end of the numnber and you will get your wind direction. POP06 is the probability of precipitation for a 6 hour period ending at the specified time. POP12 is the probability of precip for a 12 hour period ending at the specified time. QPF is the quantitative precipitation forecast.