Ars longa,
Vita brevis,
Occasio praeceps,
Experimentum periculosum,
Iudicium difficile.
English Translation appears in two forms, first easier to understand and the second one is using words with Latin origin:
The art is long,
Life is short,
Fleeting opportunity
Dangerous experiment
Difficult judgment.
Art is long,
Vitality is brief,
Precipitous occasion
Perilous experiment
Difficult judgment.
Often the phrase is taken out of context referring to the availability we should have to experiment new things when the opportunity comes along. Some occasions are once in a lifetime that is why we should take advantage of them and live intensively.