Genesis 1:5 is a verse from a special book called the Bible. It's in the very beginning of the book, at the start of the story of how the world was made.
This particular verse talks about the very first day of creation. God had just created light and separated it from the darkness. Now, he divides time into two parts: the daylight and the nighttime.
It's kind of like when you wake up in the morning and it's bright outside, you know it's daytime. And when you go to bed at night and it's dark, you know it's nighttime. God did the same thing, but on a much larger scale.
So, when the verse says "God called the light Day, and the darkness he called Night," it means that God gave names to the different parts of time. He made it so that we could know when it's day and when it's night by the light and darkness around us.
Overall, this verse helps us understand how important time is and how God created the concept of day and night.