Things that are done tacitly assume there is an unspoken understanding between the people or groups involved. Walking into your friend's house without knocking first tacitly expresses that you are like family, not a guest.
The Latin root of tacitly is tacitus, which means that is passed over in silence, done without words, assumed, silent. Leaving the seat at the head of the table for the guest of honor is a way to tacitly express respect and observe a custom. Maybe you never talked to your friends about sitting together at lunch — you just started doing it one day — you tacitly plan to continue eating together.
