what does {} and format do in this command python? [duplicate]

Hi i am reading a source and it has a line:

fn = 'total-listmix-{}-{}.xml'.format(opt.os, opt.compiler)
exept Exception as e:
    raise Exception('error merging coverage: {}'.format(e))

I think the {} mean a dict but i do not understand what dict here ? Is is a dict took from format ?