This is the badge you have to wear during 3 days that lasts the Basel fasnacht (carnival).
Basel Fasnacht is very popular in Switzerland, it's the oldest and biggest carnival.
So, you have to buy this badge before and wear it as a proof that you financially support the Fasnacht, there are 4 sorts of "plakette" and 4 different prices : copper, silver, gold and bijou (jewel).