The default for "Data Var" is "data", which would mean that a parameter called that way would receive it's content.
In the following example, the data parameter would be set to "some content":
{samplemoslate}some content{/samplemoslate}
If this field is empty, you cannot pass data to a Moslate. In that case the following two lines would be equal:
{samplemoslate/}
{samplemoslate}