Commune or Gemeinde in German is the basic administrative unit of Switzerland, usually corresponding to a city or village.
In many cantons, you would have registered your arrival in Switzerland and applied the residence permit at a communal office (e.g. contrôle des habitants, bureau des étrangers, office de la population).
However, in Geneva, due to its small size and the city-canton structure, arrivals are declared to the canton directly, at the Office cantonal de la population et des migrations (OCPM).
While your residence permit is being processed, the communal office, or in your case, the cantonal office, can issue a certificate to attest your status in Switzerland so you can apply for housing and open bank accounts etc.
In Geneva, you can apply online as described at https://www.ge.ch/obtenir-attestation-office-cantonal-population-migrations/comment-obtenir-attestation-ocpm. The certificate will be sent to your registered address (unless you have an e-service account already).
It will contain your address and attest to your legal stay or residence in Switzerland.