What does Genesis 8:15 mean?

The previous two verses showed a gap of nearly two months between when Noah saw that the earth was dry and this day. The fact that Noah waited for God's direct instruction before leaving the ark is no coincidence. Earlier, Scripture indicated that Noah was faithful and obedient to God (Genesis 6:8, 22; 7:5). Noah's survival to this point required him to trust God's commands in building the ark. Before taking the last living members of the human race out into a radically-changed world, Noah will remain patient and allow God to control his timing.

Rather than rushing out at the first sign of muddy earth, Noah waits for God to give the word that they could leave the ark. This is that moment.