The "Picardy third" translation describes the change of the third in the chord - for example, instead of ending on an A minor chord with A, C, E, with a tierce de Picardie the chord will be A, C#, E, with the minor third between the A and C becoming a major third.
Here is an example from Johann Sebastian Bach's cantata Ich habe genüg.
Before the singer begins the aria in this section, we have the continuo playing this cadence. What makes this cadence a tierce de Picardie is noted by the red natural sign: instead of this red sign being a flat, which would make the chord minor; we have a natural, making this chord major.
The device was common up to the eighteenth century.