In the "rules" step of an export a lot of things can happen to your items. You might change your item's titles, exclude items, or set the category of your items. If you want to get a good overview of which items are exported and what the data (e.g. price, title, category) for those items looks like when its being sent to the export channel, the "check rule" can prove to be useful.
To create a "check rule" go to the rules step of your channel and click "new rule". Enter a name, for example "check" and click "create". The new rule which is then created should look like this:
The rule might seem useless at first, because it takes "all" and then "does nothing". However by clicking "selected items" on the right side, you can check what your items look like at that point in the rules. If you add the check rule all the way at the end of your rules, you can always find out how your items are sent to your export channel.