Cognitive Load Theory
Renowned educationalist Dylan Wiliam (popularizer of assessment for learning) has said about Cognitive loads theory (CLT) that it is "the single most important thing for teachers to know."
When a heavyweight in the field of education such as Dylan Wiliam praises CLT, we should take heed.
Sweller's CLT is a theory with robust evidence in the form of randomized controlled trials whose ultimate goal is to make (subject agnostic) instructional recommendations from the instructional effects observed. The original research (found here) is difficult to read which is why I've chosen Oliver Lovell's book: Cognitive Load Theory in Action. It is a succinct 150 page summary of the theory and its teaching recommendations packed with practical examples too.
The Theory is a summary of the 5 pillars that support the recommendations found in Optimizing Intrinsic Load and in Reducing Extraneous Load.