Pre-heating is the process of getting the microwave oven warmed up before baking. This is the same as you would heat up the griddle before cooking chapati or heat up the oil before frying. These days, almost all new microwave ovens or plain ovens have the automated option of pre-heating where you just need to select the temperature and start the pre-heating. If you do not have this option or want to do it manually, then just select the temperature as per your recipe and start the equipment with nothing in it for the specified time. For e.g. a recipe might ask you to pre-heat the oven for 2 minutes at 400 F.