Filters in Power Query work like Excel's AutoFilter, with one big difference: they're steps. When next month's file arrives, the same filter runs again on the new rows.
Open the arrow on a column header to:
- tick or untick values to keep (a text filter);
- pick Number Filters for a condition such as "greater than or equal to";
- untick (blank) or choose Remove Empty to drop missing values;
- sort ascending or descending.
Filtering by a ticked list keeps exactly those values; a condition keeps any value that passes it, including values that appear only in future files.