According to Wikipedia:
[…] a labile verb […] (or ergative verb) is a verb that can be either transitive or intransitive, and whose subject when intransitive corresponds to its direct object when transitive.
In English, most verbs can be used intransitively, but ordinarily this does not change the role of the subject; consider, for example, “He ate the soup” (transitive) and “He ate” (intransitive), where the only difference is that the latter does not specify what was eaten. By contrast, with a labile verb the role of the subject changes; consider “it broke the window” (transitive) and “the window broke” (intransitive).
For further discussion of related topics, see the Wikipedia article.