
The triangle stands for the 3 relationships in an adoption - the biological parents, the adoptive parents, and the adopted child, which some organizations apparently refer to as a "triad". The heart weaving in and out of the triad stands for the love that ties the three relationships together.
Not all adoptive arrangements turn out to be good ones, so I imagine there will be some people (probably adoptees) who do not accept this symbol. In my case, I think these elements are particularly appropriate.
*(I grew up hating hearts because they were "girlie". So it was with great reluctance that I embraced the infinity-heart poly symbol, which I only did because, at the time, the alternative was a freaking *parrot*, which was even worse than the heart and harder to make into jewelry, and nobody came up with anything that the collective community liked better, although some of us have tried)