Print ready files first and foremost are one of two things. Vector Files or 300 DPI or higher resolution files.
Vector images, allow a high level of flexibility. These files are built using mathematical formulas rather than individual colored blocks (pixels), vector file types like EPS, AI and PDF (not all PDFs are Vectors) are perfect for creating graphics that require any type of resizing, which makes them perfect for t-shirts and posters. It allows us to scale the artwork to any size to fit any product without damaging the integrity of the design.
So if not vector, why does it need to be 300 DPI or higher? Raster images (.psd, .png, .jpg, .tiff, etc.) are made using a fixed number of colored pixels, so they can’t be dramatically changed in size without compromising their quality. When you adjust the design to fit a space they weren’t designed to fill, their pixels become visibly grainy and the image distorts, creating a muddy or blurry looking print