Cool season (or winter) vegetables are now finishing their season. Some continue to produce later than others. Eventually though, they all succumb to warming spring weather. As they do so, they relinquish their space to warm season (or summer) vegetables. Many warm season vegetables want to start growing as soon as possible. Later phases must wait for space to become available.
Later phases are no problem. They actually prolong the season for plants that are productive for only a brief season. For example, if sown at the same time, corn seed germinates and grows into stalks that produce all their corn at the same time. If sown in small groups every two weeks or so, corn seed grows into groups of stalks that produce corn every two weeks or so. That is ‘phasing’.
Phasing is more common with the cool season vegetable plants. Most of them are true vegetables, rather than fruits that are classified as vegetables. Individual plants produce only once, and can not produce again after harvest. Conversely, most warm season vegetables are actually fruits. (They contain seed.) Many of the plants that produce them continue to produce after harvest begins.
For example, squash, pole bean and indeterminate tomato plants that start growing in spring can continue to produce until frost. (Determinate tomatoes and bush beans have shorter seasons, so can benefit from phasing.) Cucumber vines can produce until frost, but might get shabby enough (from aridity) for replacement halfway through their season. Pepper and eggplant thrive in warmth.
The various greens and the various root vegetables, which are truly vegetative rather than fruiting vegetables, should grow in phases.
Seed for corn, bean, root vegetables and most greens should go directly into the garden. Seedlings do not transplant well, and are expensive in sufficient quantity. Romaine and head lettuces are exceptions that produce well from seedlings. Tomato, pepper, eggplant, cucumber and squash seedlings transplant easily. If only a few are required, they are not much more expensive than seed.