Hey there -- everything in the square brackets ( [ ] ) is an XPath filter.
I don't know if it will help, but here is how I think of those filters. Our data source could have many rows of data. We want information from one of them. Everything in the [ ] are our instructions to InfoPath for how to find the right row.
In the case above, the formula is saying:
Give me the D_Title field where the Time_Stamp is the same as this other data source time stamp, and User_Alias is the same as this other data source user alias.
In the formula editor, if you check the Edit XPath box, you'll see the full XPath, which can quickly show you what data sources are involved....Or, in the formula editor, highlight the whole mess and click the Insert Field or Group button. This should open another dialog with the field (D_Title) already selected. You can then click the filter button to see what filters have been applied.