Williams means that poems evoke ideas through the use of sense images, not the other way around. If you simply state your ideas, you will not transform language in the way required to make poetry. For example, if someone writes "I saw him in my imagination," then it's boring, there's not poetry; but when someone insists they saw him in "my mind's eye," a metaphor is created from the image of an eye that creates the idea: mind's eye is the equivalent of imagination, but much more interesting.