In Shared Hosting a common server is used by several other websites. The resources provided by this server are shared with all other websites. Various resources like Hardware, Software, Emails, E-Commerce etc. In simple words you can say that Shared Hosting is like renting an apartment with several other tenants and utilizing the services together. It is the cheapest form of Web hosting service.
The drawback of using this hosting is that if the traffic to your website increases then it may affect other websites. If one website is getting more traffic then it will utilize more space and due to which other websites are left with very less space to share which affects other websites to experience low speed and increased downtime. We can say that if there is a bad neighbor in the apartment then the owner asks him to move somewhere else.