Dimensions
Dimensions are the width and height in pixels, such as 1920 × 1080. This is what a website or form usually means when it asks for a size.
File size
File size is how much storage the file uses, in KB or MB. It depends on the dimensions, the format and the compression. A 1920 × 1080 picture can be 100KB or 3MB.
Resolution (DPI/PPI)
Resolution is how many pixels are printed per inch. It matters for printing, not for screens. A 35 × 45 mm photo at 300 dpi is 413 × 531 pixels.
Which one do I change?
Pixel size: use the resize tool. KB or MB limit: use the compress tool. Print size: choose the right passport or print sheet tool.