Equilibrium: This is when there's no net change in concentration of a substance because it's evenly distributed throughout space. It's like reaching flat ground after biking downhill.
Permeability: This refers to how easily substances can pass through a material or membrane. A highly permeable membrane lets more substances through, much like an open gate allows more people into a park.
Hydrophobicity/Hydrophilicity: These terms describe whether molecules are repelled from water (hydrophobic) or attracted to water (hydrophilic). They play key roles in determining which substances can passively cross cell membranes and which cannot.