Pertex Shield + is an improved version of Pertex Shield with higher waterproofness 20,000mm hydrostatic head (ie the depth of water that can sit on top of the fabric without it leaking) and good breathability of 20,000 MVTR (Moisture Vapour Transmission Rate measured in g/m2/24 hours). This compares with 10,000 hydrostatic head and 6,000 - 7,000 MVTR for standard Pertex Shield.
Pertex Shield + also compares well with much more expensive fabrics like Gore-tex Pro-shell which has a 28,000mm hydrostatic head and breathability of 25,000 MVTR.