Rest your head on your arm (right arm during Shacharis, left during Mincha)
Pronunciation
Rachum v'chanun, chatati l'fanecha Adonai Elokai, malei rachamim racheim alai v'kabeil tachanunai Shomeir Yisrael, sh'mor sh'eirit Yisrael, v'al yovad Yisrael ha'om'rim Sh'ma Yisrael
Meaning
Compassionate and gracious One, I have sinned before You, Lord my God. Full of mercy, have mercy on me and accept my supplications Guardian of Israel, guard the remnant of Israel; let not Israel perish — those who declare "Hear, O Israel"
About these words
We rest our head on our left arm (or right if not wearing tefillin) during the first part — a posture of vulnerability.
On Mondays and Thursdays, a longer Tachanun (V'Hu Rachum) is said.