Bonus shares are free shares that shareholders receive in exchange for shares they already own. These allotments are usually in predetermined ratios, such as 1:1, 2:1, 3:1, and so on.

Only if you held shares on the Ex-date or sold shares on the Ex-date (due to the T+2 settlement cycle) would you be eligible for Bonus shares.
For example, if the Bonus ex-date is April 10th, you must purchase the shares on or before April 9th to be eligible for the Bonus.

Dividends are subject to the same rules.
When a bonus is awarded, the share price drops by a factor based on the issue's ratio, but the investment value of the held stocks remains the same, and the leftover value is delivered to you in the form of bonus shares.
The balance amount will be credited to the registered bank account if the bonus shares eligible are not in whole numbers.

Bonus shares are normally credited to your DEMAT account 15 days after the record date, however this varies according on the RTA (Registrar & Share Transfer Agents). When your bonus shares are credited to your DEMAT, you will receive a communication from CDSL as shown below. It's important to remember that the bonus shares will be credited with a temporary ISIN and will not be available for trade right away. After receiving trading authorisation, it normally takes 2-3 days for the shares to move from the temporary ISIN to the permanent ISIN. Only after the bonus shares have been approved for trade will they appear on the trading terminal.

Bonuses and splits have essentially the same effect, but with splits, the face value of the shares decreases.