If you go to your channel there is your profile picture, channel art and thumbnails that is branding. A logo would be something people use to identify your brand, for example Pewdiepies logo is the brofist. You to have them all have a consistent design and represent who you are on YouTube. Half of being successful on YouTube is who you market yourself and branding is a big part of that. Since not everyone is skilled at graphic design they ask people on yttalk to make the stuff for them.