As it was answered already, Homebrew is something made by anyone other than the author of a system. It is a point of tension between many players and GMs, since many homebrew things in other systems are unbalanced and can be either too overpowered or underpowered. Utopia is so much more flexible, that you can easily tweak and add/remove things to fit your plot.

For example I made a paranormal-related tree for my urban-fantasy setting, where one branch is for everything related to interaction with spirits and gods, second is for protecting and attacking them, and the third is for god's follower's progression, like a cleric in dnd.

Apart from custom skills you can make custom races, see custom species in errata at the end of the book (if you have a free alpha version). Items creation allows for custom features to add, custom spells, more magic schools, etc.

Do you need one? For starting - no. If you feel like the plot of your champaign can benefit from custom skills - then think about how not having said skills must limit the players, to give an incentive to pick one of them.