'Essence' is a substantivation of 'esse' (= to be). 'Exsistence' is a substantivation of 'exsistere' (=to exsist). In English there is hardly any difference between 'to be' (= esse) and 'to exsist' (= exsistere), but in Latin (and in the exsistentialism of the 20th century) there is really a very big difference between those two terms.