A wildcard certificate secures a domain and all of its first-level subdomains with a single certificate. Instead of a certificate per subdomain, one wildcard covers them all.
What a wildcard covers
A wildcard for *.yourdomain.in secures www.yourdomain.in, mail.yourdomain.in, shop.yourdomain.in and any other first-level subdomain - including ones you add later, with no reissue needed. It covers one level only: *.yourdomain.in does not cover a.b.yourdomain.in.
A validation note
Wildcards can't use the HTTP file validation method - use DNS validation (the recommended method anyway). Wildcards are available at DV and OV assurance levels; EV does not offer wildcards by design.
Wildcard vs multi-domain (SAN)
Use a wildcard when you have many subdomains under one domain. Use a multi-domain (SAN) certificate when you need to cover several different domains. Some products let you combine both - a wildcard as one of the SAN entries.
Buying and setup
Enter *.yourdomain.in as the Common Name in your CSR and at setup. Everything else works like a normal certificate.