Bacon is trying to say that deduction is the wrong way to go about obtaining scientific truth. Deduction starts with abstract ideas or broad hypotheses and then finds evidence to support them. Bacon argues that this method of starting with axioms is inefficient. He adds that starting with an accepted fact or group of facts is useful but limited because it relies on already established facts and "idols." That is to say, deduction relies on past ideas which may or may not be true.
Instead, he argues for an inductive approach. When using induction, the thinker/scientist finds a set of particular facts or observations in order to get to a larger idea. With this method, the thinker finds evidence and then comes to a conclusion. Broadly speaking, deduction works the other way; you start with a general idea and find other instances that support the general idea.
He says that the correct form of induction would also involve the process of negative exclusion. Just as we look for particular qualities of gold, for instance, we must also look for qualities that gold does not have.
But a really useful induction for the discovery and demonstration of the arts and sciences, should separate nature by proper rejections and exclusions, and then conclude for the affirmative, after collecting a sufficient number of negatives. (CV)
Bacon uses heat as a simple example in Book II. To examine heat, Bacon notes that the scientist investigates all instances where heat exists, focusing on the most relevant ones because there are so many. Then he looks for instances in which heat is absent (the rejections and exclusions). Then he measures degrees of heat. The goal is find the true "form" of heat. There is heat in fire and in boiling water. Since the fire emits light and the water does not, light is not a necessary condition for heat to exist, so it is excluded when describing the true form of heat. This method of affirmation and exclusion is to be continued until a true form of heat is agreed upon. This is all about experimentation and observation. This method is often noted as an important step in the development of the scientific method.