Each clause in the filter control has a number of controls that are used to edit the filter.
From left to right:
The + button adds a new clause to the filter above the line clicked on (to add a clause to the end of the filter, click the + button displayed at the bottom on a line by itself).
The - button deletes a clause.
The checkbox lets you disable a clause while preserving its information (as opposed to deleting it). When a clause is disabled it has no effect on the filter at all.
The drop-down labelled And above lets you choose the Boolean operator (and or or) linking the selected clause with the one above it. The very first clause in the filter, or the first clause in a sub-clause, will not have this drop-down because there is nothing to link it with.
The drop-down labelled Name above selects the type of the clause - which attribute of the file is actually compared (name, size, date, etc). This is also used to begin a new sub-clause by selecting Subclause from the drop-down list.
The drop-down labelled Match above lets you choose whether the clause must be true (Match) or false (No Match) for the filter (or parent clause) to match.
You can use the filter control with the keyboard:
Tab to move between controls in the current clause