Tags are groups of words that describe an object, in this case a beer. Tagging helps describe the beer better, and will eventually help others. Examples of tags for beer might be the brewery name, the beer style, the color of the beer, the taste of the beer, ingredients of the beer, etc.
Simply type in your list of tags seperated by commas. You can even use two words. For example I could type in "troegs, pale ale, hoppy" and that would be 3 different tags. You must use a comma if you have two words as one tag, even if it is the only one you are putting in. For example "pale ale,". If you just put "pale ale", then two tags would be made one for "pale" and one for "ale".