The nominal voltage of a transformer is related to the turns ration between the primary and secondary. In use the primary voltage can vary and the secondary current can vary. Both these will affect the output voltage of the transformer.
To cater for varying primary and secondary conditions, transformers are often fitted with taps on one of the windings; so that the turns ratio can be adjusted somewhat. These are often expressed as numbers.
For example, a low voltage transformer may have -5%, -2.5%, 0%, +2.5% and +5% taps. At 0% tap the transformer will be operating at its designed turns. At +2.5% the transformer secondary voltage will be 2.5% larger than what it would be if set at 0% (for the same primary voltage and secondary current).
An example of use would be setting a +2.5% or +5% tap on a transformer which is heavily loaded to help compensate for the voltage drop in the cables.