These definitions should shed some light on the words to help make sense of the sentences.
Diffusion: the process in which a substance moves from a high concentration to a low concentration (think of people in a crowded room that will move to an empty room if a door opens).
Selective permeability: a characteristic of some cell membranes that allow some substances to cross via diffusion, while not allowing others (like a bouncer only letting certain people enter the empty room).
Equilibrium: a state in which all forces or gradients are balanced (everyone in both rooms have equal amount of space to move around and are happy).
Osmosis: the process in which a solvent (usually water) moves from high concentration to low concentration (just like diffusion but for solvents).
1) Most human cells have a membrane that is selectively permeable to oxygen and will allow oxygen to diffuse through the membrane down its concentration gradient.
2) Most human cells are also selectively permeable to water, and will allow water to move down its concentration until it reaches equilibrium in the process of osmosis.
I hope this helps. Feel free to message me if you have any questions.