“Vidui maaser” was a statement made at the Temple affirming that we have taken the appropriate tithes from our produce, not retaining anything that should have been given to a kohein, a Levi or the poor.
This statement was made in the fourth and seventh years of the shemittah cycle, on the last day of Passover. It could be recited in any language and, while meant to be recited at the Temple, if one recited it elsewhere, he fulfilled his obligation.
The reason for this mitzvah is that it's one thing to be negligent in tithing one's produce; it's quite another to make a bald-faced lie about it. Even those who don't treat tithes seriously would think twice before getting up at the Temple and claiming to have separated terumos and maaseros if they didn't. The need to make this statement helps ensure that people give their tithes, which is good because kohanim, Leviim and the poor count on them to survive.
This mitzvah applies in Temple times. It is discussed in the Talmud in tractate Sotah (32a-b) and in the Mishna in the fifth chapter of tractate Maaser Sheini. It is codified in the Mishneh Torah in the eleventh chapter of Hilchos Maaser Sheini. This mitzvah is #131 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #17 of the 26 mitzvos that can only be performed in Israel according to the list of the Steipler Gaon.