What does "creep up on" mean?

Advance slowly or stealthily, as in The cat crept up on the bird, or Autumn is creeping up on us. This expression is recorded in slightly different form— creep in or creep on—from the 15th century on. One of the Hymns to the Virgin and Christ (c. 1430) has “Now age has cropen [crept] up on me ful stille.”