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:
How goes the night, boy?
The moon is down; I have not heard the clock.
And she goes down at twelve.
I take't, 'tis later, sir.
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.