Predicates in propositional model:

The predicates take a number of arguments in which for now we assume are ground terms and represent a relationship between those arguments that can be true or false. Here the semantics of an n-ary predicate p(t1,...tn) are defined by a model (Δ, Θ) like in follows: if we first calculate the n objects that the arguments refer to Θ(t1), ..., Θ(tn). Θ maps p to a function P: &Delta n→{true,false} that defines where p is true for those n elements of Δ. So at different models can assign different functions P that is they can provide different meanings for each predicate. 

By combining predicates and ground terms and also propositional connectives gives us ground formulae, that don't contain any variables. We can say that they are definite statements about specific objects.