To White has yet to establish a base. Textbooks will tell you that a is the vital spot? for black to attack white, threatening b.
In an example of the proverb my opponents vital spot is my vital spot, white may well play at a to make shape:
...and this is the book continuation.
fills the marked Ko
although this sequence appears commoner at the top level. the sequence a,b,c,d follows.
was shown to me by a stronger player a while ago. Although I know it as the vital spot even now, it still looks strange to me.