Plenum spaces are open space pathways above a ceiling or below a floor that facilitate air circulation. They are generally used for HVAC systems. If you need to run cable near or inside a plenum space, Cat5e plenum-rated cable is the cable you need.
Plenum cable will not spread fire and produce black smoke as easily in case of a disaster. If you're not sure if this a requirement, check the local building codes pertaining to Plenum cable use as each jurisdiction can be different in definition and guidance.
If you are planning to run cable inside a building and through a wall, choose Cat5e CMR Riser Rated PVC Cable for this situation as it is more affordable.