m.roblox.com used to be a variant of the Roblox website that was shut down. It was the mobile version of the ROBLOX site. The website was created on August 26, 2012 and shutdown on January 24th, 2017. The website would heavily encourage you to download the Roblox App the website had an option to go to the full site.
This variant of the website cannot browse the Games page or look through the Library. Though it can browse the catalog which has been considered to be strange because the Library and the Catalog are very similar. This could possibly be a choice made by admins to keep the website smooth.
Mobile Exclusive Items
There were many mobile exclusive items you could get for free on the mobile ROBLOX site, although they were exclusive to mobile devices and you couldn't get them on PC, Xbox One, Mac, VR, etc. Many of these items included the Jack o' Mask and the Medieval Hood of Mystery. Although there was a trick PC users did to get the mobile exclusive items for free on PC. This trick only worked on Google Chrome. Users could copy the number of the mobile exclusive item on the PC site, then go on the mobile site, then go to the catalog and click on a random featured item, then go to the "buy now" button and right click on "Inspect Element", then users would replace that items number with the mobile exclusive item's number and press enter to purchase it for free on PC. This most likely caused ROBLOX to fix this patch by shutting down the "m.roblox.com" site. This also might be why ROBLOX hasn't released any mobile exclusive items anymore.