An ambitransitive verb is a verb that can be intransitive or transitive without requiring a morphological change. That is, the same verb form may or may not require a direct object. English has a large number of ambitransitive verbs. Examples include read, break, and understand (e.g., "I read the book," saying what was read, or just "I read all afternoon").
Ambitransitive verbs are common in some languages, and much less so in other languages, where valency tends to be fixed, and there are explicit valency-changing operations (such as passive voice, antipassive voice, applicatives, causatives,...