Omikuji are fortune papers from temples and shrines that offer advice for your health, career, and good luck in general. There are positive ones - marked kichi - and negative ones - marked kyou. The most positive one is the daikichi, the big fortune.
You can find omikuji at both temples and shrines. At Senso-ji temple in Asakusa you shake a box full of sticks. The stick you draw shows the number of your omikuji. Take a paper from the drawer with the corresponding number and see if you are "lucky". Other temples and shrines might give you a random omikuji or let you draw one.
If your omikuji is a positive one you may keep it. It is a thin piece of paper you can keep in your wallet as a reminder of your visit. But if you have frawn a negative omikuji, you have to folded it and tied it to the tree or line where the bad luck ones are collected. That way you leave bad luck behind at the shrine or temple and only take the good luck with you.