What does this phrase in Macbeth mean?this is malcolm talking in act 5 scene 4." for where there is advantage to be given, BOTH MORE AND LESS HAVE...

The text, as printed in the First Folio:

• 
  Seyw.
  What wood is this before vs?
  2295:
  Ment.
  The wood of Birnane.
  2296:
  Malc.
  Let euery Souldier hew him downe a Bough,
  2297: And bear't before him, thereby shall we shadow
  2298: The numbers of our Hoast, and make discouery
  2299: Erre in report of vs.
  2300:
  Sold.
  It shall be done.
  2301:
  Syw.
  We learne no other, but the confident Tyrant
  2302: Keepes still in Dunsinane, and will indure
  2303: Our setting downe befor't.
  2304:
  Malc.
  'Tis his maine hope:
  2305: For where there is aduantage to be giuen,
  2306: Both more and lesse haue giuen him the Reuolt,
  2307: And none serue with him, but constrained things,
  2308: Whose hearts are absent too.
  2309:
  Macd.
  Let our iust Censures
  2310: Attend the true euent, and put we on
  2311: Industrious Souldiership.

As stated, his own soldiers, officers and enlisted, have revolted; since both "more and less" have done so, he's left with "none" or zero soliders who truly wish to serve him.  So we've got a bit of a numbers pun here.  Those who do serve are "constrained things" meaning their (absent) hearts aren't really in it.

The text before and after Malcolm's statement feeds into this numbers theme -- "[we shall] shadow the numbers of our host" meaning the tree camouflage used by the soldiers will cause him to miscalculate the number of soldiers attacking him.  So the opposing army also has "more and less" -- "less" is the number that he thinks will attack, "more" is the actual number of camoflaged soldiers who will!  In this deception, Malcolm "gains the advantage" in the battle.