A Domain name is a unique name that many times refers to websites which can be accessed publicily. One example of a domain name is chakallas.com which is a top level domain name. A Sub Domain on other other hand is something like support.chakallas.com or sales.chakallas.com.
A Domain and Sub Domain may be represented by the same person or company and they can be a front end to different representatives. Let’s take another example of blogspot.com, the domain blogspot.com is represented by google and it allows people to register sub domains so that anybody can register a sub domain and create a blog. In this example of blogspot.com the domain is represented by google and other sub domains are represented by various other people.
Search Engines also assume that a domain is represented by different person and any sub domains are represented by some other people. Yes it may happen that your sub domain ranked high in search engine rankings and the main domain is not ranked high in search engine results. It all depends on the efforts you put in to build content, generate back links for domain or sub domain. This website will provide more examples on the differences between domain and subdomain from search engine perspective.