Things that are outdoor belong outside. Your slippers aren't outdoor shoes, but your snow boots are definitely outdoor shoes.
A picnic is an outdoor meal, and a sculpture garden is often an outdoor area of a museum. Bird watching, surfing, and snowball fights are all outdoor activities, and if you're an outdoor enthusiast you probably enjoy them more than watching TV or baking cookies. The adjective outdoor was first used in the 1740s, with the related adverb outdoors following in the 1800s.