What is meant by the statement that a Cell membrane is semipermeable?

Answer 1

This means that there are more facets to 'trans' membrane molecular movement than {merely} diffusion - which is the simplest.

The smallest of molecules pass both ways unimpeded. Medium sized bio-molecules are permitted transit via facilitated diffusion; while the largest and most biologically active bio-molecules are selectively accepted or refused at any of a Myriad of Cell Membrane structures that are known [in short] as Cell Surface Receptors.