What does it mean when a guy says your his first love?

So my boyfriend was venting to me about how he felt and he suddenly said I feel like your my first love. Out of all the girl friends I’ve had your the only one who’s been there for me. He’s been in quite a lot of relationships he was married once but divorced because they were kids. I’m not sure what he means by me being his first love.
What does it mean when a guy says your his first love?
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