The item index displays all the linguistic variables in the data file. A number says how many instances are available for each item.
If there are two numbers for an item, the first is the number of instances you supplied, the second number is the number of instances that are actually used. To see what instances were rejected, and why, go to your project page, and select data overview > errors.
Click on a number to get a data map, which displays the number of instances of the item at each site.