This move prevents a black move at
this point, which would be an ideal
extension from the corner-enclosure.]
C[White 1 is non-urgent. Black 2 is big]
LB[jq:2][do:A][jq:2]TR[co]C[White 1 is uninteresting as the left
side is small because of the low
position of the marked black stone.
(Even if it it were at A, black 2 would
still be the spot for white.)]
C[This move is not really possible
without stones at A and/or B.
Check out Ishida Yoshio's 'Dictionary
of Basic Joseki' part 1 for some
variations concerning this move.]