Click on above page and once you are there go to audit section and click on hyper-linked word "Introduction to audit"
A new page opens up. There you select the video no8 - 7 Auditing Management assertions.
I suggest you t0 go through all videos in the page.
Assertion is simply defined as "the implied representation made by those charged with governance which is usually embodied in the financial statement". Assertion in F8 includes the following - Accuracy, Completeness, Classification, Allocation, Cut-off, Occurrence, Valuation, Existence, Right and Obligation, Understanding (usually put as ACCA COVER U in an acronym).
Taking an example, A company prepared FS, they claimed it is correct on all balances, Now, the auditor don't believe that, and need to collect audit evidence to prove A's claim (i.e. assertions) is correct. There are assertions for Statement of Financial Position, P&L and Disclosure, the name of assertion appearred very similar, but we do need to categorise it into three groups.