Learn what DPI is (and PPI) and when you should and shouldn’t worry about it.
DPI is used to calculate physical measurements. Printers, for example, don’t work in pixels, so we need a way of translating our content on screen—pixel dimensions—to a printable format. Thus we have the dots-per-inch measurement, which defines the print density and bridges the physical measurement of inches with our digital pixel resolutions.
James Ritson
Understanding DPI, by James Ritson for the Affinity Spotlight blog, is probably the best, easiest to understand explanation I’ve come across. So if you’re confused between DPI, PPI, and resolution in general, give the article a read!