If Black doesn't prevent White 2, the result will be either a Seki or a Ko.
Moonprince (21k): I'm confused. Why is the one marked 'ko' a ko? Doesn't white have to play elsewhere before playing at the marked stone, and in the meantime, won't black capture the white stones at '2'? (Apologies if I messed anything up, I haven't tried to edit a diagram before.)
P7A77 (10k): It's a ko because white's play elsewhere would threaten something hopefully worth more than what black stands to keep. That's why we call 'em ko threats. :) Black would respond, at which point white could re-take, puting the onus on black to find something worthy of a white response so as not to lose his entire group.