Shakespeare is referring to something he came across in the historical chronicle he used as a reference when writing Macbeth. Remember the Norwegians we heard about in scene 2, who were attacking the Scottish forces? According to the historical account, at one point, the leaders of the two sides met in an attempt to negotiate a truce. The Scots provided food and ale, but were careful not to drink much of the ale. The Norwegians drank heartily, not knowing that the ale had been spiked with a narcotic made from the root of a plant. The Norwegians lost their senses and were easily slaughtered. Banquo is wondering if they’re somehow feeling the effects of that same plant.