Assertions

Assertions are the set of representations by a management team that were incorporated into the financial statements and accompanying disclosures that they produced. Auditors investigate the validity of these assertions as part of their audit procedures [AccountingTools]. These are characteristics that need to be tested to ensure that financial records and disclosures are correct and appropriate. If assertions are all met for relevant transactions or balances, financial statements are appropriately recorded.
Assertions are an important aspect of auditing. Since financial statements cannot be held to a lie detector test to determine whether they are factual or not, other methods must be used to establish the truth of the financial statements.
Assertions are defined as "a statement that is believed to be true by the speaker." An assertion can be anything, e.g., “I assert that fundamental value investing is the best investing philosophy”[Corporate Finance Institute].

Sources:

Assertions definition. AccountingTools. Retrieved from: https://www.accountingtools.com/articles/assertions.html

Assertions in Auditing. Corporate Finance Institute. Retrieved from: https://corporatefinanceinstitute.com/resources/accounting/assertions-in-auditing/