A phrase in a contract that means that performance by one party at or within the period specified in the contract is necessary to enable that party to require performance by the other party.

Failure to act within the time required constitutes a breach of the contract. The general rule is that time is not of the essence unless the contract expressly so provides. As a result, with respect to real estate transactions, the modern view is that time is not of the essence unless the parties have manifested such an intent. The same is generally true in construction contracts and in contracts relating to the manufacture of goods. When time is not of the essence, courts generally permit parties to perform their obligations within a reasonable time.
https://legal-dictionary.thefreedictionary.com/Time is of th...
https://definitions.uslegal.com/t/time-is-of-the-essence/

The phrase “time is of the essence” comes up often in legal matters. So what exactly does it mean?

“Time is of the essence” is a contractual term that requires timely completion of a task. If timely completion of the task does not occur, then the other party to the contract will have rights against the defaulting party.

The most common exposure that people will have to this is if they are buying and selling property in Queensland. The standard Queensland conveyancing contracts state that time is of the essence. This means that if a party doesn’t do what they were supposed to be the due date in the contract, then the other party is entitled to terminate the contract. Examples of this include if the buyer doesn’t obtain finance approval or the seller is not ready to settle by the due date. In some situations, the defaulting party may be liable to the other party for costs and compensation.

The fact that there may have been delays outside the defaulting party’s control is not considered when time remains of the essence. An example of this is if a buyer’s bank cannot arrange a valuation in time to secure their finance application. In some cases, the parties may agree to extend the due date for performance of tasks, but this has to be by mutual agreement.

Time is not always of the essence. If it is not expressly stated in the contract, then it is likely to be the case that the parties have a reasonable time to perform the task. In some other states of Australia, there is no express requirement of time being of the essence in conveyancing contracts.

If you enter a contract in which time is of the essence, you must ensure that an adequate time frame has been negotiated to allow you to perform your contractual obligations. You also need to attend to these obligations promptly.
https://dorewebb.law/blog/time-is-of-the-essence/