A Brohoof is similar to a 'Like' on facebook, Or an upvote on YouTube. When you Brohoof a post it means that you agree with it and like commend that particular post.
First of all,Welcome to the forums,!
A Brohoof here is,basically,the ponified counterpart of the Like button on most of the websites.Its most common use is when you agree with someone's post,and you want to let him know that.
Brohoof is kinda like a term used for a fist pump like action.
And if you are referring to the site's feature there are this kind of icon near every post
You can press it if you like or appreciate the content user has provided in said post or you can have your own reasons to brohoof certain content.
After you've pressed the button you can undone it to prevent mistakes but if you refresh the page you can't do that anymore.
Brohoofs are shown in your userpage on the right hand side in the green box
kanashimi purinsesu's post in just joined the fourms was marked as the answer