This FAQ, begun in August 2020, will eventually be the place to come for documentation about the Android version of CrossWords. I'll be adding to it as I see the need. A great way to demonstrate need is to ask questions :-).
The Wordlist Browser lets you quickly scroll through all the words in a CrossWords wordlist. But sometimes you want to see a subset of the words, perhaps because you're curious (maybe you're studying three-letter words) or because you're looking for a high-scoring move. Filtering lets you limit the words shown to those that match a filter you create.
Three lines labeled Starts with, Contains and Ends with are where you put the tiles you want to filter for. If a field is left empty it's ignored. Fields are combined: you'll see only those words that match all of them. Also, they can overlap. So if, for example, you ask for "Starts with: RI" and "Ends with: ING" you'll see "RIDING" as well as "RING" because they match both.
You can use blanks to match any tile. For example, "B_U" will filter for words with a "B" followed by any single tile followed by a "U".
The blank button lets you insert an underscore character ("_"), which represents a blank ("wildcard" in pattern-speak.) You can do this from the keyboard too, but the button makes it easier.
The two dropdowns near the bottom of the filter settings area let you choose the minimum and maximum lengths of the words you'll see. If you're memorizing three-letter words this is where you'll filter for them.
This checkbox causes the tiles listed to the left to match in any order. So if you are looking for "Ends with" and the tiles "ING", words ending in "GIN" will be included.
When you press this button the settings changes you've made are checked, applied to your wordlist, and remembered so that you'll see the list in filtered form, even after restarting CrossWords, until you change them.
If you've used a shortcut, e.g. a lowercase letter, you'll see it replaced by the upper-case spelling of the tile once the filter is applied.
All filtering, including lengths, is in terms of Tiles, not letters. In English they're the same, but in many languages some tiles have more than one letter, e.g. CH, LL and RR in Spanish. In such languages if you're looking for words of length 5 you'll get some with five tiles but more than five letters. It's a feature, not a bug.