The Committee of Public Safety was a governmental organisation that sprang up after the French Revolution in 1789. It was created in 1793 as a executive force to steer France during the chaos of the Revolution, and to prevent attacks from foreign powers and from internal counter-revolutionaries.
The Committee was under the leadership of Georges Danton from April 1793 until July, when Maximilien Robespierre was elected leader. Under his watch the Committee became extremely powerful, effectively running the government. It began to arrest its enemies and suppress public criticism of its authoritarian style of government. Many people were condemned to death for their failure to support the Committee. Robespierre and his allies used their position of power to suppress their political rivals on the Committee for General Security, trying to keep hold of power. Eventually, however, they lost control, and were arrested in turn; Robespierre was guillotined on 28th July 1794.
All things considered, Alexander I's comparison of his Privy Council to this Committee is massively inappropriate. Firstly, the Committee very swiftly degenerated into a paranoid, murderous group aimed at clinging on to power by any means; and secondly, it was created out of the regicidal atmosphere of the French Revolution. For an autocratic emperor with no real intention of reforming his state to claim any association with this Committee even 'in jest' shows how out of touch with reality he had become.