Explain what Macbeth means when he says, "Things bad begun make strong themselves by ill" (III.ii.56) in Macbeth.

This quote is actually the final line of a speech in which
Macbeth delivers something of his own incantation, similar to Lady Macbeth's speech in
Act One, scene five ("Come you spirits/That tend on mortal thoughts...).  He calls not
on spirits, but the night itself to use its "bloody and invisible hand" to assist the
"deed of dreadful note," which is the murder of Banquo and
Fleance.

The fact that he is not just being metaphoric, but
actually summoning evil is supported by line 53:

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Whiles night's black agents to their preys do
rouse.

He concludes with the
quote you have given, a statement which insinuates that now that he has summoned the
"black agents" of night, that they will take matters into their own hands, and the
outcome, whatever chaos this might involve, is up the actions of these forces.  And thus
the deeds themselves which are "bad," add unto themselves, without further aid from
Macbeth.