What do mean by minimum subscription? What happens when a company is unable to collect the amount of minimum subscription?

Minimum subscription is the application for a minimum number of shares the company needs to get from the public out of the total issue by the date of closure. (Presently, as per the Companies Act, every company needs to raise minimum 90 per cent of the issued amount before allotment). This has been put in place to prevent companies from commencing business without adequate capital. 

If applications received for the shares are for an amount less than 90 per cent of the issue size, the allotment cannot be made and the application money received must be returned to the applicants.