For example, when looking up the night sky, I can spot more stars in my peripheral vision. But some of them disappear when I look at that very spot directly.
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Your eyes have two types of light sensitives cells, rods, and cones. Rods are more sensitive to light but only show the brightness, not the color. Cones are for color vision but are less light-sensitive than rods.
The central part of the vision contains more cones than the rods so you can see color bur require more light. The peripheral vision contains mostly rods so more light-sensitive but does not have a lot of colors.
The result is that dim light sources might be visible in the peripheral vision but not the central vision.
Evolutionary the function of peripheral vision is as a warning system to spot stuff that might be dangerous and color vision is not as important as low light vision. Detecting motion is often enough without really knowing what it is.
The central vision is used for stuff where color is more important like for example searching for fruit, something to hunt etc. We are mostly day active so good color vision in the day is an advantage.
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