what is crop?
A 100% crop means you cut or "crop" a section of an unprocessed image (the unselected region is "discarded") so the resulting image is small enough to occupy 100% of your monitor without resizing though of course monitor resolutions differ so there is some ambiguity in the definition.
Images can be resized to hide/reduce noise and softness, thereby giving the false perception of improved image quality but a 100% crop can give a truer indication of sharpness and quality.
how cam I upload the full file without losing quality?
I'm not bashing PhotoBucket but have you checked alternative image hosts? I don't know about PhotoBucket but some of the alternatives do allow the images you upload to remain as you uploaded them, without reduced quality or resolution.