first, this is specific to Tixati therefore belongs in the Help and Support section. It is not "Off Topic"
Second there is now a Search feature on the forum and in fact there are LOTS of results for "trading"
Other than that, most of the information you seek is on the normal support pages, https://support.tixati.com/bandwidth%20view
Basically, trading is a feature to set a reserve for reciprocating with a user you are also downloading from, to facilitate giving preference to someone who is giving you pieces.
Seeding is just giving freely. Trading is a reward you can decide to give to those giving to you.