Meringue, a fluffy dessert or dessert topping, is comprised of egg whites and sugar. The egg-sugar mixture is whipped into a froth. Sometimes a meringue pie recipe develops moisture between the filing and meringue. Different factors can cause this.
Adding a 1/4 tsp of cornflour and a tablespoon of cold water and mixing it into the meringue stiffens it. This keeps it from sweating, which causes slippage.
Keeping the meringue at room temperature while the pie filling cooks helps keep it from sweating later. This is because cool filling on hot filling, and vice versa, creates a condensation effect.
Putting a meringue pie in the fridge or freezer before it has cooled to room temperature causes it to sweat. Furthermore, not covering it securely can also cause moisture to form between the meringue and filling.