The MCS index value list gives every combination of "number of spatial streams + modulation type + coding rate" that is possible. In practice the achievable MCS value will depend on a large number of variables, and it may be prudent to run your hardware at a lower MCS value on purpose in order to allow for adequate fade margin in your link. The MCS value will only tell you the 'over the air' data rate of a link and not tell you what the actual usable throughput will be. You will need to refer to the documentation of your hardware for this information.
MCS INDEX VALUE
This is the unique reference given to the combination of "number of spatial streams + modulation type + coding rate".
Using MiMo technology it is possible currently to run up to 4 spatial streams. Effectively it allows you to use the same area of frequency space to transmit and receive multiple streams of data. Eventually with later revisions of 802.11ac, this will be increased to 8 spatial streams.
Modulation is the method by which data is communicated through the air. The more complex the modulation, the higher the data rate. More complex modulations require better conditions such as less interference and a good line of sight.
This is an indication of how much of the data stream is actually being used to transmit usable data. This is expressed as a fraction with the most efficient rate being 5/6 or 83.3% of the data stream being used.
The Guard Interval (GI) is effectively a very short pause between packet transmission to allow for any false information to be ignored. Longer Guard Intervals make for more reliable wireless.
This is how much of the channel is being used, the maximum being 40MHz. The table only shows values for 20MHz and 40MHz, but other values may also be used, and the throughput can be calculated by using the shown values as a baseline. For example, a 10MHz channel will provide half the Data Rate of a 20MHz.