It’s not so much a slang as sort of an emoticon denoting: “Le positif amène le positif“, as opposed to “Le négatif attire le négatif“.

Si vous voyez tout en noir, la négativité n’apporte que de la négativité. En revanche, avoir l’esprit positif vous permet d’entrer dans un cercle vertueux, dans une spirale positive, dans laquelle, par exemple, une réussite entraîne une autre, ou encore votre attitude positive déteint sur le monde qui vous entoure.

Seeing the responses I feel compelled to give you maybe an other light on this matter.
We often disregard the influence of the things we don’t know or don’t understand.
It was a saying, to say "le positif attire le positif" as the opposite of "le négatif attire le négatif". But it became more used and PARTICULARLY on the french YouTube because of Lena Situations. She’s a french YouTuber, one of the biggest actually. She said that so much that it became her brand. "+=+" appears now on the line of clothes she was a part of with the brand Jennifer, and even is the title of her book. Because of her and her influence, it is now used as a way of expressing good feelings in general I guess. Hope it helps.

if this is surrounded with a circle, it means "zero + zero = la tête à toto"