Irony is common in English, especially in humour. When the speaker or writer says one thing but wants you to understand something different, they are being ironic.
Sometimes the implied meaning is the opposite of the words being used, or the person could be trying to be rude, even though the words used are seemingly polite etc.
Example of irony:
Your friend turns up in ripped jeans. With a smirk, you say, "I see you have put on your best clothes!"
Category: Figure of Speech