What is "to tread upon the heels of"

Tread \Tread\, v. i. [imp. Trod; p. p. Trodden, Trod; p. pr. & vb. n. Treading.] [OE. treden, AS. tredan; akin to OFries. treda, OS. tredan, D. & LG. treden, G. treten, OHG. tretan, Icel. tro?a, Sw. tr[*a]da, tr["a]da, Dan. tr[ae]de, Goth. trudan, and perhaps ultimately to F. tramp; cf. Gr. ? a running, Skr. dram to run. Cf. Trade, Tramp, Trot.]

1. To set the foot; to step.

   Where'er you tread, the blushing flowers shall rise.
   --Pope.

   Fools rush in where angels fear to tread.
   --Pope.

   The hard stone Under our feet, on which we tread and go.
   --Chaucer.

2. To walk or go; especially, to walk with a stately or a cautious step.

   Ye that . . . stately tread, or lowly creep.
   --Milton.

3. To copulate; said of birds, esp. the males. --Shak. To tread on or To tread upon.

   1. To trample; to set the foot on in contempt. ``Thou shalt tread upon their high places.''
      --Deut. xxxiii. 29.

   2. to follow closely. ``Year treads on year.''
      --Wordsworth.

      To tread upon the heels of, to follow close upon. ``Dreadful consequences that tread upon the heels of those allowances to sin.''
      --Milton.

      One woe doth tread upon another's heel.
      --Shak.