Key Maturity in refers to the date T in the future that defines the time interval over which the stochastic evolution is described by the volatility here.
The exact definition of the time interval depends on the choice defined within Vol Ref.
For example, assume the vol is quoted to have the value 20% and the choice equals Black
Then the relevant time interval is the whole period from the T₀ until T with the volatility 20% being the vol of a constant-vol lognormal difussion from T₀ until T
Another possibility would be a so called "local volatility" type, where the relevant time interval is the infinitesimal period (T, T+dt) and the volatility 20% relates to the stochastic evolution during that infinitesimal period only.