One or two dots (. or ..), OK, but what do three or more dots mean?

Why does this work?

if [ -d "." ] ; then echo "ok"; else echo "nok"; fi
if [ -d ".." ] ; then echo "ok"; else echo "nok"; fi
if [ -d "..." ] ; then echo "ok"; else echo "nok"; fi
if [ -d "...." ] ; then echo "ok"; else echo "nok"; fi
if [ -d "nosuchdir" ] ; then echo "ok"; else echo "nok"; fi

Output:

ok
ok
ok
ok
nok

What is supposed to mean ... or .... in a shell env?