In this scene, the Council of Venice is in deliberation about the war with the Ottoman Turks. They agree to send Othello to fight the Turks, at which point Brabantio (a member of the Council himself) arrives to accuse Othello of using some sort of trickery or even witchcraft to seduce his daughter Desdemona:
She is abused, stol'n from me and corrupted
By spells and medicines bought of mountebanks;
For nature so preposterously to err,
Being not deficient, blind, or lame of sense,
Sans witchcraft could not.
In short, Othello's marriage to Desdemona is so unnatural (partly because he is a black man, a Moor) that she must have been under some sort of spell to agree to it.
Othello responds by saying that he is not a great speaker, but that his love for Desdemona, and hers for him is legitimate. He promises to tell the Council, and Brabantio, how it happened, which he later does. He sends for Desdemona herself to vouch for the sincerity of their relationship, and she promises that she truly loves Othello. So the lines in question are essentially Brabantio's public protest against his daughter's marriage to Othello.