Sowing "under cover" or "under glass" means growing seedlings in a warm, light environment that replicates the normal growing conditions for the species sown. This can be in a warm greenhouse, heated propagator or bright windowsill.
Growing under cover provides many advantages, such as an early and extended cropping season for vegetables, improved quality and quantity of seedlings, and less dependence on the weather.
Seeds need moisture, enough light and the correct amount of warmth to germinate and grow. If its cold, wet and windy outside you can provide these conditions artificially, by placing your seed trays (pots or modules) in a warm a place such as a sunny windowsill or heated propagator.
Sowing seeds directly outside in the ground is of course the most natural and easiest way to grow plants. However, for tender species, this would not be possible until the outside temperature warms up enough. But if you wait until the conditions are right, there will not be enough growing time for plants to mature and flower or bear fruit. Therefore starting seeds off early, under cover, will help them get away quicker and extend the effective growing season.
Even hardy seeds sown in the grown have a lot to contend with in the first few weeks, such as late frost, snow, rain, wind, slugs, snails, and birds. So certain hardy varieties can also benefit from early indoor sowings too.
Seeds can be started early on windowsills (with a sunny aspect), by a patio door or in a conservatory. If you have a heated greenhouse, even better. Although a greenhouse can be an expensive option and not really necessary unless you are growing a large number of seeds. If you just have a couple of seed trays then a sunny windowsill in the house works equally well.
Take care that young seedlings don't overheat or dry out on a windowsill in strong sunlight. An east or west facing window is usually best. Where they will only get direct light from the weaker sun in the morning or evening. Even then, you can start them off with a plastic cover in the airing cupboard, then move them to a well lit window, conservatory or greenhouse as soon as they have sprouted. It also pays to make sure the seedlings are not kept too warm or too dark, or they will become leggy and drawn. When growing on a windowsill, turn the seed trays occasionally, or they will tend to grow to one side, towards the light.
After about four to six weeks, once the seedlings are growing away strongly and outside conditions are more temperate, you can start to hardened them off (acclimatised to outside conditions) and plant them outside in the ground or in containers.