hello udit, to assert means to say, management through financial data

1. transactions
2. assets & liabilities
3. presentations

also asserts some critical things

if we see sale of 50,000 (transaction)
it asserts occurance has happened, classification was proper, accuracy, completeness, cutoff (ise saal ka agle saal nahi aur pichle saal ka ise saal nahi)

so we have to collect evidence about these assertions whether it really exists.

similarly we have assertions for assets & liability and presentation & disclosure

see video of core audit concept and SA 315,