Why are some processes highlighted in top?

Solution 1:

To expand on Doug's answer, look at the states of the processes. All but the three highlighted processes are in state S, and the three highlighted processes are in state R. The process states are given in section 3a, item 20 of the top manpage:

20. S  --  Process Status
   The status of the task which can be one of:
       D = uninterruptible sleep
       R = running
       S = sleeping
       T = traced or stopped
       Z = zombie

   Tasks  shown  as running should be more properly thought of as
   'ready to run'  --  their task_struct is simply represented on
   the Linux run-queue.  Even without a true SMP machine, you may
   see numerous tasks in this  state  depending  on  top's  delay
   interval and nice value.

To toggle that highlighting, press y:

y :Row-Highlight toggle
  Changes highlighting for "running" tasks.   For  additional
  insight into this task state, see topic 3a. DESCRIPTIONS of
  Fields, the 'S' field (Process Status).

  Use of this provision provides important insight into  your
  system's  health.   The only costs will be a few additional
  tty escape sequences.

Solution 2:

The 3 highlighted processes are the 3 running processes.