This is the 1086th most frequent Spanish word.
seres
"Seres" means "beings" in English.
In this sentence, 'seres' is used as a noun meaning 'beings', specifically referring to humans ('human beings').
Los seres humanos han desarrollado una gran variedad de culturas.
Human beings have developed a great variety of cultures.
Here, 'seres' refers to 'beings' as a category of entities, specifically describing their immortal nature.
En la mitología griega, los dioses eran considerados seres inmortales.
In Greek mythology, gods were considered immortal beings.
In this context, 'seres' again refers to 'beings', in this case implying extraterrestrial entities.