They demonstrate various methods that children learn at school to help them add, subtract, multiply and divide. Some very early methods, such as those learnt in Reception and Y1, are not shown here.
The early parts of the videos show a lot of informal jottings to help mental maths. Before children tackle formal written calculations (seen later on in videos), they learn to use jottings, or notes, to help them calculate mentally. They support children’s understanding of the ideas behind the maths they’re doing, before taking those ideas on to more traditional methods (which are more ‘abstract’).