It's a bit of a long read, but I think the best introduction is still this [0] and the comments were here [1]. Yes, it's presented in the context of rust and gamedev, but ECS isn't actually specific to a particular programming language or problem domain.

[0]: https://kyren.github.io/2018/09/14/rustconf-talk.html

[1]: https://news.ycombinator.com/item?id=17994464