An overhear means to listen to someone's conversation without them knowing you're there.
When people talk about overhearing something, they usually mean that they were nearby when two or more people were having a private conversation. Sometimes this happens by accident - maybe you were walking down the hallway and caught snippets of their chat, or perhaps you were sitting in a quiet café and overheard some strangers' conversation. It's often awkward if someone realizes you've been listening to them talk about something they didn't mean to share.
(ambitransitive) To hear something that was not meant for one's ears.
Example: I was hanging clothes in the garden and I overheard the neighbours talking about Sheila's pregnancy.
