The verb contain has many shades of meaning but it often describes something that's held back or held in by something. Kids on the last day of school may find it hard to contain their glee when summer officially starts.
Picture Times Square on New Year's Eve. People can barely contain — or hold back — their excitement at being there. They clutch cups that contain — or hold — hot drinks that help them stay warm. The police set up barriers to contain — or restrain — the crowd to keep everyone under control. The next morning, the heaps of trash on the street will contain — or include — streamers, party hats, and cups.