Hello,
A brain lesion is any type of abnormal tissue in/on the brain, which is picked up on an MRI or CT scan. They can be caused by lots of things, including infections and changes in the blood vessels. They can sometimes be harmless or can be caused by something more serious that will need treatment.
I can't find any information that says Humira definitely causes brain lesions when I do a quick search, but there are a couple of things popping up which suggest that IBD can actually result in some brain changes in rare cases, but these aren't necessarily reliable.
I would suggest having a chat with your mum and asking her if your doctor gave her any other information, I don't know how old you are but in the UK we have a right to know whats going on even when under 16, provided we have the capacity to understand.
I would suggest also writing down any questions you have and asking your doctor when you next see him. In the mean time hopefully someone will be along soon who will know a little more about this, try not to google it though as it'll come up with all sorts of unlikely but scary possibilities.