Macduff tells Malcolm of the terrible situation in Scotland. What is learned of the situation in ACT 4, SCENE 3 of Macbeth?

There are two situations discussed in Act IV, scene iii --
the public and private.  The private situation discussed is the murder of Macduff's
family because of the charge of treason that Macbeth has laid upon Macduff's head.  To
which charge, Macduff says simply, "I am not treacherous."  And Malcolm answers, "But
Macbeth is."

This leads to the discussion between the two
about the public situation, a situation totally controlled by Macbeth.  The two lament
the state of affairs in Scotland, recognizing that the murder of Macduff's family might
well be the straw that breaks the camel's back, not just because it is a heinous crime
against Macduff and his wife and child, but because it is the act of a tyrant.  Macduff
says:

Bleed,
bleed, poor country!

Great tyranny, lay thou thy basis
sure,

For goodness dare not check thee.  Wear thou thy
wrongs,

The title is affeer'd!  Fare thee well,
lord,

I would not be the villain that thou
think'st

For the whole space that's in the tyrant's grasp.
. .

Malcolm
agrees:

I
think our country sinks beneath the yoke:

It weeps, it
bleeds.  And each new day a gash

Is added to her wounds. .
. .

When I shall tread upon the tyrant's
head,

Or wear it on my sword, yet my poor
country

Shall have more vices than it had
before,

More suffer and more sundry ways than
ever,

By him that shall
succeed.

And it is Malcolm
who will succeed Macbeth.  The conversation of this scene finally bends towards this. 
And though, Malcolm pleads a modest inadequacy to assuming the throne of Scotland, by
the end of the scene, Malcolm and Macduff have agreed to ally with England to win back
their Scotland from the tyrannical hold of Macbeth.