FT3 is a full text indexer for datas stored into a sqlite3 database. Full text means that the text fields stored into the database can be split word by word and stored in a form suitable for lookup. You can then create easy SQL queries for searching for cells from yours indexed tables containing the words from the query.
How to read the documentation for the stable's version V0.3
If you don't know how full text indexing works, read the howto part of the documentation.
If FT3 is good enough for you as is then read the user guide.