The "local" means the garden variety of computing standard deviations, i.e.statistical, from the number of events in the resonance peak, plus systematic error, which is now added in quadrature.
"Global" is a way of trying to estimate the Look Elsewhere Effect ( LEE). The words say it, when one is scanning a large phase space for something unusual it is not only the statistics of the specific mass region where a fluctuation has been observed that gives the significance but possibly the statistics of the whole region where one has been blindly searching.
If the higgs really exists at a specific mass then necessarily once enough statistics is accumulated there, there should be the given local significance. Since one does not know if and where the higgs is, it is inevitable that a statistical fluctuation might simulate a real signal, if one looks elsewhere in enough phase space.