Clean vs Unclean

Genesis 7:1-4
1 The LORD then said to Noah, "Go into the ark, you and your whole family, because I have found you righteous in this generation. 2 Take with you seven of every kind of clean animal, a male and its mate, and two of every kind of unclean animal, a male and its mate, 3 and also seven of every kind of bird, male and female, to keep their various kinds alive throughout the earth. 4 Seven days from now I will send rain on the earth for forty days and forty nights, and I will wipe from the face of the earth every living creature I have made."

What does it mean by clean and unclean animals? Anyone?