What resolution means for a digital image
Resolution gets used for three related ideas. Cameras quote it as megapixels, the total pixel count. Websites and forms quote it as pixel dimensions, such as 1920 × 1080. Printers talk about pixel density, the DPI or PPI that maps pixels to inches. This checker shows all three, headlined by megapixels: width times height divided by one million, so 4000 × 3000 is 12 MP.
Is my image high resolution enough?
It depends on where the image will end up. A full HD screen is 1920 × 1080, about 2.1 megapixels, so a photo that fills it edge to edge doesn't need more than that. A 4K screen is 3840 × 2160, about 8.3 megapixels.
For print, work backwards from the size you want at 300 DPI. A 6 × 4 inch print needs 1800 × 1200 pixels (2.16 MP). An 8 × 10 inch print needs 2400 × 3000 pixels (7.2 MP). The checker lists the print size at 300 DPI for your file, so you can see straight away whether it is big enough.
More pixels doesn't always mean more detail
The pixel count tells you the maximum detail a file can hold, not how sharp it actually is. A blurry or heavily compressed 12 MP photo can look worse than a clean 3 MP one, and enlarging an image with a resizer creates new pixels by interpolation rather than recovering detail.
Likewise, the DPI value is only a label. Changing it from 72 to 300 alters the print size the file suggests but leaves the pixels, and therefore the real resolution, exactly as they were.