When a clause in a contract (normally related to a time deadline) is made "of the essence", it means that the innocent party can claim immediately if the relevant provision is broken without having to give any notice or allow a period for compliance. This is often used where the aggrieved party wants to be able to terminate a contract for non-compliance rather than merely claiming can only normally claim damages for losses he suffers due to the delay. If a term is not of the essence, a new deadline would need to be set which would then be made of the essence.
Easyform Glossary of Law Terms. — UK law terms.