A camera is not as smart as a human eye. Your camera is like a person suffering from Alzheimer's disease. You have to say: "hello camera, this is white". Then the camera says: "well, if this is white, than this must be red, this dark red, and this light red, and then this is blue...". If you don't do this whenever the lighting situation changes, the camera gets confused.
Imagine the following: You take a nice landscape picture where the sun is your light source. You have green grass in it and a white sheet of paper somewhere in the picture. If you set the white balance correctly, the paper appears plain white and the grass green.
Now you take this sheet and bring it inside your house. You lower all the blinders and the only light source you have is a bulb. Now the same sheet appears rather yellowish (warmer), due to the warm color temperature of your light source. What you have to do now is, show the sheet of paper to the camera and tell: "The light in this room has a different color temperature than the light outside and white looks now like this." If you don't do this, the colors in the final picture will appear very warm, so that the sheet looks very yellow and people's faces even more yellow.
Fortunately, almost all cameras can be set to automatic so they figure this out by themselves whenever you take a picture. Some cameras are good in this, some are less. Most cameras also have presets for different situations.
In case you want to know what I do: I don't take pictures in JPEG format but in RAW. The RAW format allows me to set the white balance after taking the picture.