In the "rules" step of an export a lot of things can happen to your products. You might change your product titles, exclude products, or set the category of your products. If you want to get a good overview of which products are exported and how the product data (e.g. price, title, category) for those products 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 products" and then "does nothing". However by clicking "selected products" on the right side, you can check what your products 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 products are sent to your export channel.