Page, Channel and Post ID's are unique identifiers. They are used to link Pages, Channels and Posts to Page Widgets.
Where can I find them?
Channel ID can be found by going to a channel and taking getting it from the URL bar. It is called the workspace#.
Page ID can be found by going to a page and taking getting it from the URL bar. It is called the pages#.
Post ID can be found by into a channel and finding the post you are looking for. You will then need to click the three dots in the top right corner of the post and select view post. You can then get the post ID from the URL bar. It is called discussion#.
Article ID can be found in the same way as finding the Post ID.