Many yards and gardens have partial shade conditions due to obstacles that limit the amount of sun available to flowers and other plants. Trees can create filtered light, where some sunlight comes through, but not enough for plants that require full sunlight. Buildings and fences create shadows that prevent the adjacent flower beds from receiving sunlight during parts of the day.
If your garden has part shade, consider including some of the annuals that do well in such conditions. Some annuals, such as Browallia, or Sapphire Flower, do well both in full sun and partial shade areas. You can select plants like these if you're not sure about the conditions.
This page lists annuals that do well in part shade, grouped by color. It also includes listings of plants that can flourish in multiple types of light conditions. Check with your garden center on which annual flowers will thrive in your location.