Cards can represent different things in different processes. In a task management pipe, each card represents a task. In a sales pipeline, each card is a new sales opportunity.
To create a new card in a pipe, first you need to click on the "+" button (either in the lower right corner of the pipe or next to the first phase's name) to open the start form:
After you do that, fill in the form's fields and hit the button at the bottom to create a new card.
Cards can also be created using:
Note: As a default, new cards are created in the first phase of the pipe but you can edit the other phase's settings to enable users to create cards in other phases as well (the "+" icon will be displayed next to the names of the other phases you enable card creation in).