This is the 308th most frequent Portuguese word.
deles
"Theirs" (masculine or mixed group).
In this sentence, 'deles' indicates possession related to 'they' referring to a masculine or mixed group.
Os livros deles estão na estante.
Their books are on the shelf.
Here, 'deles' shows possession of 'presentation', implying it belongs to 'them', referring to a masculine or mixed group.
Eu fiquei impressionado com a apresentação deles.
I was impressed with their presentation.
In this context, 'deles' means 'of them', connoting ownership or association with a masculine or mixed group.