Fides: Faith, trust, loyalty, or confidence.
Et cetera (etc.): “And the rest,” “and so on,” or “and other things of the same kind”.
The Reformers that were teaching the tenets of the Reformation were teaching ‘sola fides’ – faith alone. Today that teaching has devolved into Fides et Cetera – faith and some other stuff.
But the reformers had it correct, and those who would add performance to the gospel are falling into error.