By design, you will usually work on one activity at a time in many applications. However, with some applications it is more natural to work on several activities in parallel. For example, in the middle of writing an e-mail, you may want to return to the inbox to look up another message or read mail that just arrived.
webOS handles activities by assigning a seperate card for the draft e-mail and the e-mail inbox. You can have as many cards as needed in this respect; each is considered a seperate activity and independently accessible.