Porosity is the measure of the hair's ability to absorb moisture. This is determined by the condition of the hair's cuticle layer (the overlapping scales of the hair shaft), and is rated as low, normal, and high.
In normal, healthy hair, the cuticle is compact and inhibits the penetration of the hair shaft by moisture - both moisture going in, and moisture coming out. When the cuticle is overly compact and prevents the penetration of the hair by moisture it has low porosity.
Hair with low porosity is harder to process, and is resistant to haircolor and perms. Low porosity hair must usually be softened prior to other chemical services. Hair with high porosity is hair whose cuticle layer is open and the hair too-readily absorbs moisture.
Overly-porous hair also releases moisture easily and becomes dry and is easily damaged. Acid-balanced conditioning treatments are used to contract the cuticle layer and lock-in moisture on overly-porous hair.
Extreme caution must be used when performing chemical services on overly porous hair to prevent damage. Haircolor will take much more quickly and strongly in overly porous hair (and semi-permanent and demi-permanent haircolors will fade much more quickly). Perms and straighteners will process much faster in high-porosity hair as well.
To determine the porosity of your hair, simply feel it - both when wet and when dry. If the hair feels straw-like and rough when dry, or if it feels gummy or slightly rubbery when wet, then you probably have a problem with overly porous hair. Or you can carry out a hair porosity test.
See also: More on how to Analyse Hair