A fixed expense is a recurring expense with a predictable amount: rent or mortgage, subscriptions, insurance, fees, basic utilities. It typically charges automatically and does not change month to month except for occasional reviews.
A variable expense, on the other hand, depends on how the month goes: eating out, transport, leisure, clothing. This is where most people have room to adjust without cutting essentials.
To design your financial cushion, the right calculation is: how many months can I cover my fixed expenses with no income. That is the foundation of the emergency fund.