This is the 693rd most frequent Portuguese word.
delas
"Delas" means "of them" or "their" (feminine).
In this sentence, 'delas' is a possessive pronoun that refers to a group of females who own the car.
O carro delas é muito bonito.
Their car is very beautiful.
Here, 'delas' indicates that the idea ownership belongs to the group of females mentioned earlier.
Elas fizeram um projeto, e o resultado foi ideia delas.
They made a project, and the result was their idea.
In this example, 'delas' specifies possession or relation, indicating that the friends belong to the group of females being referred to.