Lock-in indicates a freeze on the sale of shares for a certain period of time, imposed by SEBI mainly to ensure that promoters continue to hold a minimum percentage in the company after a public issue.
Source: Chapter 3, Issue of Shares
The examiner expects two key elements in this answer: (1) the meaning — a freeze/restriction on selling shares for a fixed period, and (2) the purpose — to ensure promoters retain a minimum stake post-public issue. Both points together make a complete 1-mark answer. Do not write more than one or two lines.