Intransitive verbs do not have a direct object. They can be compared to transitive verbs, which need one.

For example:
'I lied', 'She is crying', 'We left on Tuesday' are all examples of intransitive verbs.

In the classroom
Students can explore which verbs can be both transitive and intransitive; they are verbs that are often used to talk about some kind of change. E.g. 'The door opened' and 'He opened the door'. Other examples include empty, increase, shut, spoil.

Further links:

https://www.teachingenglish.org.uk/article/word-family-framework

See also: 

https://www.teachingenglish.org.uk/article/transitive