The earliest surviving transcriptions of The Buddha's teachings, as first written down some ~400 years after his death (prior to which they were maintained in an oral tradition). It's a very very large collection of scriptures, arranged in various groups and volumes.
Seriously — think several shelves on a bookcase, or something analagous to the many many books in those old-school encyclopedia collections.
Generally, when people refer to "The Pali Canon", they're referring to the second of its three main 'sections': the "Sutta Pitaka", i.e. the discourses of the Buddha. This is because it's overwhelmingly the largest part of the Pali Canon, and contains all the good stuff.
More details on these 'sections' below under the pitaka terminology sub-heading.
As soon as you start to dip your toes into the world of the Pali Canon (and basically any kind of Buddhist scholarship, even at a high-level), you'll start coming across these Pali-Canon-specific terms.
So, do yourself a favour and skim the below and (even briefly) to get an entry-level understanding what these terms are and how they relate to one another.
Firstly, "The Pali Canon" is obviously the English translation, but the original name(s) still used widely - these are the "Tipitaka" (Pali) or "Tripitaka" (Sanskrit). That's good to know - these are all the same thing.
The Pali Canon is comprised of three pitakas (literally 'baskets', named after the receptables used to store the original palm-leaf manuscripts), a.k.a. three main sections or categories for the content within.
These three pitakas (sections) are:
'Volume', or 'collection'. A nikaya ****is a meaningful grouping of suttas (below).**
This chiefly concerns the second pitaka (the Sutta Pitaka), as a way to group the literally 10,000+ suttas therein. There are five nikayas (collections) in this second pitaka (section).
(Pali) - you'll also come across sutra which is the same thing but in Sanskrit.