A protoplanetary nebula (PPN) is neutral gas around an asymptotic giant branch star that is not yet sufficiently hot to ionize the gas. When the central star shrinks and reaches sufficient temperature, it emits enough ionizing radiation to transformed the PPN into a (glowing) planetary nebula (PN). The term preplanetary nebula is sometimes used instead, to reduce confusion.
The term protoplanetary nebula can confuse because the words "planetary", "protoplanetary" and "nebula" suggest a relation to planets, planet formation, the nebular hypothesis and protoplanetary disks. The term protoplanetary nebula has indeed occasionally been used in that respect.