Definition & Meaning of "wear one's heart on one's sleeve"in English

What is the origin of the idiom "wear one's heart on one's sleeve" and when to use it?

The idiom "wear one's heart on one's sleeve" originated from medieval jousting tournaments, where knights would wear their lady's favor on their sleeves to publicly display their affection and devotion. The phrase is mostly used to emphasize transparency, vulnerability, and a lack of emotional restraint or inhibition in expressing oneself.