Question: According to Android definitions, I've found that:

* Resolution: The total number of physical pixels on a screen
* Density: Based on the screen resolution, the spread of pixels across the physical width and height of the screen.

How can I understand better the differences between the two definitions?

Answer: 
Resolution is a measure of how many pixels a display can show at a time eg. 640x480. Density is a measure of how big each pixel is in actual units. This relates heavily to screen size eg. 640x480 on a 15' display. 

When comparing one device's resolution to another, it's a straight-forward comparison: the higher the resolution, the more information you can display at a time. When comparing density: the higher the density the sharper the display will appear. 

If you have high resolution and low density, then your screen will be very large. On the other hand, low resolution and high density will result in a very small screen.
**Resolution** is the number of pixels on a device found in each dimension (width × height) that can be displayed on the screen. 

*For example,* a device with the resolution of “1024 × 768” has a 1024-pixel width and a 768-pixel height.

**Pixel Density** is usually measured in PPI (Pixels Per Inch), which refers to the number of pixels present per inch on the display. A higher pixel density per inch allows for more sharpness and clarity when using the device.

*For example,* If a device screen is called "100 PPI" then it means that it has 100 psychical pixels per inch.

[Reference](https://blog.prototypr.io/making-sense-of-device-resolution-pixel-density-40922aeb8a6)