It is Latin for "I give and you give". It is probably referencing the way in which Roman gifts/sacrifices to their deities was used. The devotee would give the gods something and in return the gods were expected to give something back.
tu as des frères et des soeurs means 'do you have any brothers or sisters' - you can reply for instance: je n'ai ni frère ni soeur (I have neither brother nor sister) j'ai un (deux, trois, quatre)...