Redundancy means there is a reserve. If one connection, power supply or switch fails, the other takes over. Users notice little or nothing.
In a commercial building it is mainly about the internet connection. A second connection, preferably entering the building via a different route (see dual entry) and possibly from a different provider, prevents one damaged cable from taking down the whole building. A lighter form is a 4G or 5G backup connection.
Redundant fibre is part of the highest IT-Label level, IT1+ Premium.
What does this mean for the IT-Label?
Redundancy is explicitly mentioned under Connectivity, and backup connections count under Infrastructure. Redundant fibre is in the official description of IT1+ Premium. It is often one of the key steps from IT1 to IT1+. See dual entry and 4G or 5G backup connection.