Imagine the brain being a computer-like device, but much more advanced. There are certain processes going on all the time, whether we are aware of them or not. While dealing with new information, the brain decides "automatically" which parts of information get our attention, and then turns our perception there. So, in order for students to learn, their brain must be able to attach personal sense or meaning to the information provided.
The immediate questions that our brains make automatically, in order to store the data, are: Does it make sense? Is it consistent or opposite with what is already stored? Is it relevant? Because when the brain dumps incoming data, it’s gone forever. We don't have even a faint memory ever hearing about the thing that was dumped.
Typically there has been very little meaning associated with traditional teaching and instruction, where every student is presented with exacty same information, whether they already knowi it or not (and whether they have the prerequisite skills & knowledge to have grounds for understanding it). This type of teaching easily causes an “I’ll never need to know that!” –thought, and /or feeling in students, AND thus forgetting. Same ideas and items cannot be interesting and meaningful for each and every student, so individualizing teaching is the first answer.
The brain must see a future use of the data or it will dump it. So we really need to make sure the taught things are relevant to our students, or, being teachers, we must "create" that relevance. Of course, while creating interest it is important to remember that things directly related to the learners’ life are the first ones to be perceived, and anything too hard or too boring will automatically be dumped by the brain.
Also, when the brain stores data, it’s permanent, although neural pathways to access data may be lost, and then retrieval is very hard. Establishing multiple pathways to the data ensures longer access to the data (this is why bi/multilingualism is so beneficial: more connections to same items and concepts). Making instructional “sense” to the learner provides these different neural pathways automatically, as it helps students become interested in the taught subject and engage with their personal learning process to make connections with their previous knowledge and the new information.