Fibre sends data using light instead of electricity. As a result, much more data can pass through, over greater distances, and the signal is not affected by electrical interference.
For commercial buildings, a fibre connection is the foundation. Without fibre, a higher IT-Label is difficult. But note: "fibre available" often only means that the cable runs to the connection point. How the signal then travels through the building is at least as important.
Read more in Fibre in a commercial building: what should you look for?
What does this mean for the IT-Label?
Fibre falls under Connectivity, where the type of connection, capacity and stability are assessed. It is the foundation of the higher levels: from IT2 Plug & Play upwards, the official description mentions a fibre connection, and IT1+ assumes redundant fibre. See also dual entry and bandwidth.