This is the 6310th most frequent Portuguese word.
dem
"Dem" is a contraction of "de" + "um" or "de" + "uma," meaning "of a" or "of one."
Here, 'deles' as derived form indicates possession or association ('their experience').
Eu gosto das aulas dos professores, pois aprendo muito com a experiência deles.
I like the teachers' classes because I learn a lot from their experience.
The word 'dos' indicates possession or relation (the students' ideas).
As ideias dos alunos foram apresentadas na reunião.
The students' ideas were presented at the meeting.
'Dos parques' shows the relationship, meaning possession ('the areas of the parks').