If I install the website on my current web hosting platform, which already has a website, will it install as a subdomain
If not, what will it do to the current website

If you install where your current site is, it will likely overwrite your current site. Kind of depends on what you have now and what your goal is.
Are you planning a completely new site with Backlight, or did you want to use Backlight just for your galleries?

If you want it to install in a subdomain, first create the subdomain (in your host’s C-Panel) and upload the files there.

Thanks Rod
Yep - A subdomain will do it

Let us know if you have any questions!