Foreshadowing and imagery, and metaphor are literary devices used by Shakespeare to add drama and often irony to his plays.
Foreshadowing appears early in Othello and helps to sow the seed of doubt in Othello's mind as to Desdemona's faithfulness. It is just this kind of circumstantial evidence that adds to Othello's insecurities. Brabantio unwittingly contributes to the eventual downfall of Othello:
Look to her, Moor, if thou hast eyes to see:
She has deceived her father, and may thee
His warning which is expressed more out of the hurt he feels than any real belief that his daughter would betray her husband
foreshadows Othello’s later murderous suspicion
and his insecurities get the better of him "... for I am black, And have not those soft parts of conversation..." By this time we are beginning to see the jealousy emerge as Desdemona drops the handkerchief which will unravel the plot and contribute to the tragic end. Emilia's need to "please his (Iago's) fantasy" foreshadows the complexity of the "evidence" that will become crucial to persuading Othello.
Iago, when talking with Roderigo, himself in love with Desdemona, reminds him that "we have reason to control our raging motions" but ironically, it is just this very weakness in man that will later prevent Othello from using reason to control his own "raging motions."
The animal imagery which appears throughout Othello whilst it is not permitted in terms of eNotes rules to address it separately, can be mentioned here in terms of how it helps reinforce the foreshadowing of Othello's descent into what can only be described as an animal tendency towards basic instinct.
Iago's constant references are intended to debase and reduce the characters, especially Othello, and add to the foreshadowing of Othello as he is reduced to nothing more than a savage beast, as it were, when he kills the defenceless Desdemona and even purports to do it to save her from herself. Iago calls Othello a "black ram" who is "tuppng your white ewe," leaving little to the imagination.
The foreshadowing continues as other characters reduce themselves to animalistic tendencies. Cassio admits to being "bestial" and even Othello , showing a complete lack of respect for the woman he supposedly cherishes, talks about "goats and monkeys" in reference to Desdemona and Cassio.
The cycle is complete and the foreshadowing confirmed when, at the end, Roderigo calls Iago " a damned inhuman dog" but unfortunately it is already too late.