This is the 576th most frequent German word.
robert
"Robert" is a proper noun, a German given name, equivalent to "Robert" in English.
In this sentence, 'Robert' functions again as a proper noun and is an object of the preposition 'über'.
Ich habe über Robert gesprochen.
I spoke about Robert.
Here, 'Robert' is the direct object of the verb 'kennen', once more a proper noun referring to a name.
Kennst du Robert?
Do you know Robert?
Here, 'Robert' is used as a proper noun referring to a person's name.