It is encouraged, if your financial situation allows for it, to pay back the deferred mortgage payments. Interest accrued during the deferral will otherwise be added to the principal amount of your mortgage and may significantly increase interest costs over the life of your mortgage.
If you cannot pay back the deferred mortgage payments in full, you can get back on track sooner by exercising your pre-payment privileges which allow you to increase your mortgage payment or make a lump sum payment. Check your mortgage financing agreement to determine your allowable prepayment privileges.