The terms are a little obscure. Cardinal numbers are what you'd consider "normal" numbers: "one", "two", "three", etc. They're used for the number of things in a collection. {mei} converts a number into a selbri that asserts that the cardinality (the number of elements) in its arguments (a mass, a set, and the elements themselves, in that order). So {le bende cu mu mei} is saying "the team has five members".

Ordinals are the numbers that indicate where in a sequence something is: "first", "second", "third", "last", etc. {moi} does a similar thing to {mei}, but it creates an ordinal selbri. That is, it tells you where in a particular sequence an element occurs.