Reading width × height
Dimensions are always written width first, then height. A 1920 × 1080 image is wider than it is tall (landscape), 1080 × 1920 is portrait, and equal numbers mean a square. The checker shows the pair in pixels as its headline, with the megapixel count beside it.
Pixel dimensions are what most requirements are really about. Profile pictures, website banners, video thumbnails, marketplace listings and online application forms usually state a minimum or exact size in pixels, and an image that is too small gets rejected or looks soft once it is stretched to fit.
The aspect ratio behind the numbers
The aspect ratio is the shape of the image. The checker reduces the dimensions to the simplest whole numbers, so 4032 × 3024 becomes 4:3 and 1080 × 1350 becomes 4:5. When the dimensions are within 1% of a common ratio but don't reduce neatly, it names that ratio instead: 1366 × 768 is reported as 16:9. Anything else is shown as a decimal such as 1.85:1.
Knowing the ratio tells you whether an image will fit a slot without cropping. A 4:3 phone photo placed in a 16:9 video frame will either be cropped top and bottom or get bars at the sides.
What the dimensions mean for print
Divide the pixel width and height by 300 to get the size in inches at 300 DPI, the usual target for photo prints. The checker does this for you: a 3000 × 2000 image prints at 10 × 6.67 inches. For a bigger print at the same sharpness you need more pixels, and raising the DPI number in the file does not add any.