Cognitive load refers to the demands on working memory during problem solving, thinking and reasoning (including perception, memory, language, etc.).
Most would agree that people learn better when they can build on what they already understand. But the more things a person has to learn in a short amount of time, the more difficult it is to process information in working memory.
The notion of cognitive load has been used by Sweller and colleagues (see below) as the theoretical rationale for designing or choosing between instructional methods such that more learning is achieved when the method reduces extraneous cognitive load. For instance, interleaving worked examples between problem solving activities has led to better learning and is claimed to do so because the worked examples relieve the extraneous cognitive load experienced during problem solving because of the need to store problem solving goals and subgoals.
See this blog for some critical perspectives on Cognitive Load Theory.
Sweller, J. (1988). Cognitive load during problem solving: Effects on learning. Cognitive Science, 12(2), 257-285.
Sweller, J., van Merrienboer, J. J. G., & Paas, F. G. W. C. (1998). Cognitive architecture and instructional design. Educational Psychology Review, 10(3), 251-296.
Sweller, J. (1999). Instructional design in technical areas. Melbourne, Australia: ACER Press.