To a small extent, you also see the "passing of the torch" in this scene. Banquo handing over duty to Fleance is foreshadowing the events of Banquo's death - which will permanently transfer manhood of the family to Fleance.

The scene foreshadows what is to follow. The night is dark; Banquo says "there's husbandry in heaven, their candles are all out." In Shakespeare, when things are well with the weather, they are also well with the world. On this night, the night of Duncan's murder, the stars are not shining; the night is in darkness, as is the King's world soon to be.

Banquo feels heavy and restless; he's been having dark thoughts, he asks for help fighting the thoughts(he asks the Merciful powers to hold back in him the "cursed thoughts that nature gives way to in repose"). It is unclear here whether he is thinking of commiting murder himself or whether he is simply agitated and spinning his thoughts as he tries to rest. The words "cursed thoughts" seem to suggest he is thinking, at least, of murder.

If this is the case, then this scene contrasts directly with Macbeth's following scene, for while Banquo asks the heavens to help him steer clear from sin, Macbeth does not (perhaps this is why he is so vulnerable to his wife's feminine wiles?).

Fleance is Banquo's son. Banquo has had the boy act as lookout in the courtyard while King Duncan sleeps. Banquo is very tired and does not wish to fall asleep himself.

Here are the lines from the beginning of that scene,(2.1.1-9) with a few glosses to aid understanding of important words:

BANQUO
How goes the night, boy?

FLEANCE
The moon is down; I have not heard the clock.

BANQUO
And she goes down at twelve.

FLEANCE
I take't, 'tis later, sir.

BANQUO
Hold, take my sword. There's husbandry (*thrift) in heaven;
Their candles are all out. Take thee that(*some piece of clothing or armour) too.
A heavy summons lies like lead upon me,
And yet I would not sleep: merciful powers (*summons to sleep),
Restrain in me the cursed thoughts that nature
Gives way to in repose!

Enter MACBETH, and a Servant with a torch

Give me my sword.
Who's there?