Essential expenses are the ones you have to pay (housing, food, bills, transport). The usual rule is 3 to 6 months; aim higher if your income is variable or your household has a single salary.
The target fund equals your essential monthly expenses times the months of cover. Current cover shows how many months of expenses your savings already pay for.
The months to reach it assume you save the same amount every month and do not touch what you have already set aside. Keep the fund in a safe, instant-access product (a savings account or a no-penalty deposit).
Educational estimate, not financial advice. The number of months of cover is a guideline; adjust it to your situation.