Putting this quote in the context of what is being said in the opening part of the poem helps us understand Eliot's meaning:
April is the cruellest month, breeding
Lilacs out of the dead land, mixing
Memory and desire, stirring
Dull roots with spring rain.
Winter kept us warm, covering
Earth in forgetful snow.
April is the cruelest month because it is when the earth experiences its rebirth. Plants begin to grow and new life emerges. April and rebirth become a metaphor for an individual's or a society's growth and rebirth.
Growth is painful, as any psychologist will tell us. It takes work and facing up to unpleasant realities. It means having to make changes. As Eliot notes, "Winter kept us warm." It is easier to sleepwalk through life and forget the past and abandon trying to change. Death and stasis are always a temptation.
But more importantly, Eliot is speaking to Western civilization as a whole, and here, the dating of the poem is important. Eliot published the poem in 1922. World War I had ended a few years before. This devastating conflict shook most Europeans to their core, traumatically undermining their faith in progress and civilization.
Eliot is saying that it is painful to have to face the realities of that war and the need for change and rebirth in Western values and ways of thinking. Eliot is, in this passage, acknowledging the allure and comfort for a society of going back to sleep and trying to forget what happened after a painful incident—but that will not solve anything.