dpi refers to dots/inch/ This value is determined by your printer.
ppi refers to pixels/inch. This value can be adjusted readily in Photoshop Elements. The rule of thumb for printing is that it is desirable to have the resolution in the 240-300px/in range, although I have printed successfully at a lower value. For web work, 72px/in is ok.
As for the prompt which you reference relating to less than 300 dpi, this shows up from time to time. Simply ignore it and proceed to print.