DPI, PPI and pixels
A digital image is a grid of pixels, and that grid is the only thing that holds detail. DPI (dots per inch) originally described how many ink dots a printer lays down. Inside an image file, the DPI field is really PPI, pixels per inch: a note telling print and layout software how many pixels to fit into each inch of paper.
Because it is only a note, the same photo can carry 72 DPI or 300 DPI without looking any different on a screen. Monitors and phones ignore the value and display pixels one to one, scaled by the device.
The print size formula
Print size in inches equals pixels divided by DPI. A 2400 × 1800 photo at 300 DPI prints at 8 × 6 inches. The very same file tagged 72 DPI suggests 33.3 × 25 inches, yet contains exactly the same pixels. Multiply inches by 2.54 for centimeters.
The checker shows the stored DPI for both directions, plus the print size at 300 DPI, so you can see whether the pixel count supports the print you have in mind.
When the DPI says not set
PNG files only store resolution if the app that saved them added it, and many JPEGs from phones, screenshots and web exports store no meaningful value. In that case the checker shows Not set (72 assumed), because 72 is the traditional default most software falls back to.
If a print shop, publisher or application form insists on 300 DPI, check the pixel dimensions first. When there are enough pixels, the DPI Changer can write 300 into the file without touching the image. When there aren't, a higher DPI number will only make the print smaller, not sharper.