When Macbeth and Banquo encounter the three witches on the "blasted heath," these strange women hail Macbeth as the future king of Scotland and then hail Banquo as the sire of an entire line of kings. (Act I, Sc. 3) When the witches' predictions about Macbeth come true, he becomes overwrought with envy, jealousy, and fear of of his former good friend Banquo. In a soliloquy in Act 3, Sc. 1 he says:
They hailed him father to a line of kings.
Upon my head they placed a fruitless crown
And put a barren scepter in my grip,
Thence to be wrenched with an unlineal hand,
No son of mine succeeding. If 't be so,
For Banquo's issue have I filed my mind;
For them the gracious Duncan have I murdered,
Put rancors in the vessel of my peace
Only for them, and mine eternal jewel
Given to the common enemy of man
To make them kings, the seeds of Banquo kings.
When Macbeth says he has given his eternal jewel to the common enemy of man, he is saying that he has sold his soul to the devil.
In Act 3, Sc. 3, acting on Macbeth's orders, three murderers attack Banquo and his son Fleance. Banquo is killed but Fleance manages to escape in the dark. So Macbeth remains obsessed with the fear that Banquo's heirs, through Fleance, will become kings.
In Act 4, Sc. 1, Macbeth goes to visit the witches again, mainly to ask for confirmation of their prediction that Banquo will be the father of a line of kings rather than Macbeth himself. They present "a show of eight kings, the eighth king with a glass in his hand, and Banquo last" as ocular proof that Banquo's male heirs will indeed inherit the Scottish throne.
The three witches prophecy that Banquo's 'seeds',that is some one from his future generation will become a king.It is presumed that the broad hint is about King James I of England,who was suposed to have descended from Banquo's lineage.