DPI (or dots per inch) is a way to measure the resolution of a printed digital image. The higher the DPI, the higher the detail and quality of an image. For most print projects, 300 DPI is optimal.
When an image is enlarged, its DPI and resolution become reduced so it’s best to use an image that is 300 DPI at actual size or larger than it will be used. This also means that you can’t “size up” or take an image of lower quality, say 150 DPI, and make it 300 DPI. (Note: vector images don’t apply to these rules as they can be enlarged without losing quality.)
To learn more, read Creative Bloq’s post, The ultimate guide to image resolution.