I'm having trouble understanding what it means.

What it means is that whoever wrote the request probably doesn't have a clue how JPEGs are rendered using most current image viewing applications.

PPI (pixels per inch) is the way we refer to how many pixels of a digital image should be rendered per inch of physical page space when the image is embedded in a document intended for printing, such as would be produced by a page setting program. Other than that, the PPI/DPI number in an image's EIXF information is pretty much meaningless.

What they should be telling you is the minimum number of total image pixels in width and total image pixels in height they require.

Having said that, it is pretty easy to set the PPI at 300. For best results, so that the images are ready to go right out of camera, set it in camera if your camera has an option to set ppi (some cameras may call it dpi, but it really means ppi when it's the camera EXIF tag you're setting). If your camera doesn't allow that, then use an image editing program to change the value in the image metadata. Then save a full resolution version with 300 dpi selected in the output settings (which is actually 300 ppi, even though the standard EXIF tag is often labeled as dpi).

If you're using JPEGs straight out of camera with no additional editing, then you can use an EXIF editor to change the value without having to decompress/recompress the image.

Please see this answer to How do I generate high quality prints with an ink jet printer? for a very thorough explanation of the actual relationship of ppi to dpi. PPI is pretty much irrelevant for viewing and printing full page photos with most current applications.