An Ecological Framework for Educational Policy, Research, and Practice in Multilingual Settings
Biliteracy – the use of two or more languages in and around writing– is an inescapable feature of lives and schools worldwide, yet one which most educational policy and practice continue blithely to ignore. The continua of biliteracy featured in the present volume offers a comprehensive yet flexible model to guide educators, researchers, and policy-makers in designing, carrying out, and evaluating educational programs for the development of bilingual and multilingual learners, each program adapted to its own specific context, media, and contents.
You have a definition of biliteracy right there.
Continua simply means that there is somehow a gradation in situations described as biliteracy, that situations in the world (different language community practices of biliteracy) can fit the description more or less clearly.
Hornberger’s edited volume claims that it provides a framework in which different kinds and intensities of biliteracy can be made sense of. My experience of edited volumes is that they do no such thing—they’re excuses for different contributors to write different snapshots of what they’re interested in, and if you’re lucky one contributor will be tasked with somehow bringing it all together, and instead end up doing a glorified summary.
Maybe in this instance they actually have formulated a framework. They said ecological framework, which suggests they’re at least looking at contextual factors to differentiate different kinds of biliteracy.
But OP, you have the book, not us. You’ll have to take it from here.