A topic is something articles generally have. The topic and the title of the article may even be the same, though this is optional. A topic might be a banana, a rabbit or any number of things or a magical mirror from a another world that has summoned the Lich King to your will. Whether it makes sense or not, or whether it is funny, varies greatly from wiki to kiwi.
This article, most likely, has a topic as well, though that remains to be seen.
These usually don't make sense, preferring to make squirrels and daisies. It may also be unrelated to the title, or to anything else. There may also not be a topic, that role being filled by chocolate sauce or a similar gooey substance. The chocolate sauce is there to attract the topic, as the intention is that when a suitable topic arrives it will eat the sauce and decide to stay. Sometimes this works. Sometimes, however, the topic drowns and the other topics all get scared and the article just never has a topic. This results in it not making very much sense, which is arguably a good thing.
This topic you are reading now happens to be in Illogicopedia, which may or may not imply something about its nature.
These usually make sense, but lack content. Squirrels are avoided in favour of potatoes. An advantage to this strategy is that article topics are more edible; a disadvantage is that they may also be rotten or green. It is advised to exercise caution when attempting to eat Uncyclopedia article topics as they smell of bad taste and humor.
These usually make more sense than the other two kinds, to the extent that neither squirrels nor potatoes are allowed, and they sit outside the door wishing they could come in and start hording the good jokes. If they ever do try to enter Wikipedia, they can expect nothing good at all. This makes them sad if they are the sort who care about that sort of thing, which they often are not.