I am wondering what is the role of CQ5 and Hybris here. In what ways it helps in managing a eCommerce application?
hybris is a framework to build ecommerce applications. CQ (AEM) is a CMS and DAM so it's not dedicated to ecommerce at all.
hybris also contains a CMS but CQ CMS is more advanced.
For me CQ is a good addition to hybris to manage pages that will be often contributed. The tool is more easy to use for someone that is not from the SI side. Also it offers more possibilities than hybris CMS.
Being an iOS developer, how far I should know about them?
It depends on what you're supposed to do.
I think CQ is not so hard to learn, at least for basic usage.
For hybris, without the intention of being mean, I would never hire an iOS developer. Indeed it's heavily based on Java EE technologies and Spring. So if you have no background in these techno you'll be quite useless...
Even Java developers I worked with experienced troubles when it comes to advanced development on the hybris platform.
If your role is only restricted to call WS exposed by hybris then you don't need to know anything about hybris. Ask the hybris development team to give you a documentation of the WS your supposed to call and that's it.