When do you use somos in spanish?

'somos' = 'we are' from the spanish verb 'ser' (to be)

'Ser', in contradistinction to 'estar' (also 'to be'), is used for more enduring, as against temporary 'being',

thus a group of Spaniards could say:

Somos espanoles = We are Spaniards (because it's not a temporary state)

but:

Estamos en el cine = We are at the cinema (Not, one presumes, permanently)