Meaning: Top-down view
A bird’s-eye view means A view from above. Birds are typically designed to be flying creatures, and when they fly, they see things from a higher angle.
People have historically been land-bound and thus look at things from a ground-level perspective. However, some things are hard to fully comprehend looking at them straight on. It’s hard to understand the layout of a city from the side. Unless you walk down all the streets and mentally map everything out, all you will see is a wall of buildings. However, a bird can fly above any obstacle and see the whole city and its layout at once. Thus, a bird’s-eye view is a view from above which can see the whole picture.
Typically this means literally seeing from above, either from a flying object or just somewhere tall. Often this is in reference to a drawing or photograph. Before the invention of airplanes, people rarely saw places from above. Thus, a drawing or photograph from above was a rare and interesting perspective. Such perspectives are still appreciated today, but are much easier to produce and more often just used for utilitarian purposes.
However, the phrase can also be used in a metaphorical sense. Like a bird’s-eye view can let you physically see the whole layout of a city, you can be said to have a bird’s-eye view of a project if you have an overall view of the whole thing, instead of just seeing one part like some working “on the ground”.