“Hear, hear!” is an exclamation that functions as a kind of verbal applause. It is a way of showing that you approve of what the speaker is saying, mostly in a formal context with many people present, such as a wedding toast, a political meeting, or a government assembly – especially in the British Parliament. Think of it as a short way of saying, "Hear him!" or "Listen to what she is saying!" It sounds old-fashioned to most people.
"Hear, hear!" is usually written with a comma between the two words and an exclamation point after them.
I hope this helps.