Monthly fixed instalment is the term often used in mortgage documentation for what is more generally known as the monthly repayment: the amount of principal and interest a borrower pays each month on a loan. The two terms describe the same underlying figure, with "monthly fixed instalment" emphasising that, on a fixed-rate loan, this amount stays constant for the life of the loan even as the mix between principal and interest within it changes.
The phrase appears most commonly in mortgage lending, where the monthly fixed instalment is calculated at closing and set out in the loan documents so the borrower knows exactly what will be due each month until the loan is repaid or refinanced.
For how this amount is calculated, what can cause it to change, and how it fits into the structure of a loan, see the main Monthly Repayment entry.