Most options in the upload page actually do nothing or are broken, for one reason or another.
Those options not listed here can be safely ignored.
Of course where you select the file to upload. So far only PNG, JPG and GIF formats are supported.
Video formats can probably be uploaded too, but be aware that it can break any time.
In particular if the video is too long don't upload it here, but provide a link to the original source. It's fine to upload preview images in this case.
The URL pointing to the page where you got the file. If it's an imageboard (e.g. 2chan) where threads are deleted after a while, leave it blank or write "Image Board". If it's from pixiv, use the address to the gallery page, not the image itself, even for single images (e.g. https://www.pixiv.net/member_illust.php?mode=medium&illust_id=69416243 as source for post #74892)
If the file is related to some other work, place that work here. As a rule of thumb, if you are going to use this feature to connect more than two files, use a pool. The ID is the number shown in the address bar, e.g. 74892 for post #74892.
Use this only for relations between 4 pictures maximum. If a picture has more than 4 children (i.e. a total of more than 4 related pictures) use a pool.
Refer to howto:tag.