In the four-level hierarchy that characterizes a MoStack the backgrounds are the second level.
A background is basically a set of fields and controls. A card always belongs to a particular background, therefore carries all the fields and controls defined in the background and allows you to enter values on the fields.
"Background" may seem to be a somewhat unusual term, and maybe something like card type, kind of card or card definition would be more familiar to new users, but on the one hand already MoStacks' inspiration HyperCard uses this term, and on the other hand it really fits the concept of a stack of cards, with the field definitions as the static backgrounds of cards and the actual field values as the "foregrounds".
A stack can have as many backgrounds as needed. If you view a MoStack as a database this means that a single stack is not restricted to a single table, but can hold any number of tables.
A background does not care, so to speak, about how many cards use it. It's perfectly ok to define a background and just use it for a single card, indeed this often makes sense, e.g. for something like a cover card for a stack that will be unique.
There is no limit for the number of fields and controls per background, but as cards do not yet scroll if larger than the phone screen it does not make much sense to have too many visible fields and controls in a particular background.