Closest packed means that the atoms are packed together as closely as possible. The FCC unit cell is actually made of four cubic close packed layers (click to show the unit cell with layers).

The first layer of atoms pack together as close as possible. (click to show layer "A"). The next level of atoms settle into the crevices between the first row of atoms. (click to show where the atoms of layer "B" fit on top of layer "A") (click to show layer "B") The next layer fits into the crevices of layer "B", but there are two possibilities. One possibility is to place the atoms directly over the atoms of layer "A". The other possibility is to place the atoms over the the second set of crevices in layer "A" forming a third layer, layer "C". (click to compare where the second level "A" and layer "C" fit on top of level "B") If the crystal packs in repeating "AB" layers the crystal is a hexagonal close packed structure (hcp); a cubic unit cell does not result. (click to show second level "A") If the crystal packs in repeating "ABC" layers the crystal is a cubic close packed structure (ccp); the result is a face centered cube. (click to show the atoms of layer "C") Highlight unit cell

Another look at SCC, Bcc, and FCC unit cells.