You could think of the sites that share your server as your roommates; there's really not that much separating you from them. Sure, you can close the bedroom door, but they can still cause nightmares for you in the kitchen and the bathroom. In web hosting terms, all the sites share a single server's resources, so huge traffic spike on Site A may impact the neighboring sites' performances. It's even possible that another site could take down the shared server altogether, if it crashed hard enough.
Telling someone what I do for a living is always an interesting experience. Either we’re totally in sync, both lost in conversation about WordPress woes or some time-saving program update, or it’s me talking with crickets in response. There’s just something about web hosting. It’s hit-or-miss whether someone is up to speed on the nuances of all that this industry has to offer.
BigCommerce is a bit different from our other hosting plans in that it's a SaaS (software-as-a-service) provider instead of an IaaS (infrastructure-as-a-service) provider. In other words, rather than renting space on a virtual machine where you setup and configure your own site, BigCommerce provides you with an app you login to that creates and online store.