Immediately below the three dots in the Photoshop CC 2018 Toolbar, you’ll see two color squares (the colors you see in the squares will vary).
Note that one square rests on top of the other. If you click on the arrows to the top and right of the squares, the colors in each square switch places.
The color on top is the foreground color, and the color under it is the background color. You can change either of these colors by double-clicking on the color square and adjusting the color in the Color Picker window that pops up.
It’s important to understand what foreground and background colors are as you learn the rest of the tools in the Toolbar.
For instance, any time you paint with the Brush Tool, you will paint with the foreground color by default. Any time you remove part of a non-layered image (by using the Eraser Tool, or by moving or deleting part of a selection), the background color will show through into your image.