Collections are like video playlists. You may create and share as many collections as you like. To add a video to a collection you should first play the video. Below the video player you will see a button called "Collections" and when you click this for the first time you will be asked to create a new collection by inserting a title and description. When you click "Create" the video you were watching will be added to that new collection.
When you want to add an additional video to the collection go to the video page for the video that you want to add and click the same "Collections" button. This will allow you to add the video to the collection previously created.
Collections are accessed by clicking on your icon in the top right corner and selecting "My Profile". You may then click on the "Collection" tab.