Again, a simplification. Firms might save by not distributing all profits to the stockholders. Government might save. But we can make the simplifying assumption that all saving is done by income recipients. It does not affect our analysis in any way and it makes the diagram much easier to read.
Saving is non-spending , thus it is a leakage from the flow of expenditure.
Investment is, of course, an injection into the flow of expenditure. Some demand leaks into other economies as imports. If this page is garbled you need a newer browser. They are free.