Poetic licence is the freedom poets take to bend ordinary rules of grammar, spelling, meter, or literal accuracy when doing so strengthens a poem. It can include unusual word order, compressed expression, invented language, or deliberate departures from fact for emotional or artistic effect.
It is not the same as a mistake: the poet chooses the departure knowingly and usually for a clear poetic purpose.
Example
The moon wore silver boots
and crossed the rooftop slow,
though everyone could see
that moons do not walk or glow in snow.