I'm not sure what it means for the cell membrane to be semi-permeable

It means that the cell membrane allows certain substances to pass through and not others. As the cell membrane consists of a phospholipid bilayer with a hydrophobic core, only small uncharged hydrophobic molecules can pass through the cell membrane e. g. glucocorticoid and other larger charged particles require transporters or channel proteins to pass through the cell membrane, e. g. glucose.