Since income can’t be taxed before it's earned, so the terms financial year & assessment year were coined. A financial year is the time period wherein income is earned, the assessment year is the following year where tax evaluation is done for the income earned in the previous financial year.
Example: If an individual starts earning in the year 2020, that year shall be known as the financial year & in 2021 his/her tax shall be calculated, which shall be known as assessment year.