On the Limit Items tab you can limit the items that are displayed in the grid.
There are a few general options, like limiting the total number of items, displaying only items that have an image or setting an offset, basically skipping the first x items.
You'll get a lot more options on the Specific Rules tab.
After enabling Limit items you can add as many limit rules as you want. These are the options available for limiting:
Use to limit the grid by item author.
Include/exclude items that were published before, after or on a specific date.
Limit by exact Post ID.
You can limit by any taxonomy associated with the post types in the grid (think categories, tags, ...).
Apart from these limit rules, you can also use the Dynamic Limiting feature.