Season's Greetings is an English label borrowed from the idea of a seasonal greeting. In K-pop it is used for a yearly release built around the coming year, usually put out toward the end of the current one. Fans often say the name in full or shorten it, and either way the name on its own tells you very little. It does not describe the contents, so you cannot work out what is inside from the title alone.
So what is it, usually? A physical package organised around a calendar for the next year, often with additional printed or photographic items alongside it. The exact contents, the format and the number of versions are different for every release, and they are only settled by the official announcement. Treat any description of typical items, including this one, as a general picture and not as a statement about one particular release.
A trailer for one of these packages announces that the item exists and is meant to build attention for it. The post that carries a trailer is often short, and it may not give contents, prices or a sale date. The details usually follow in a later, separate announcement, so a trailer alone is not enough to plan a purchase. When the fuller announcement arrives, look for the sales window, where it is sold, whether there are different versions and what each one lists, and whether the shop ships to your country. Shipping and sales channels depend on the shop and are stated by the shop.
It helps to know what this product is not. A photocard is one printed card, and an album is a music release. A Season's Greetings package is its own product, usually organised around a calendar year, and it is not a music release. Photocards, pre-save and pre-order wording, release-size labels and concept films each have their own guides, so this one stays on the package itself. One honest caveat: there is no fixed definition, so two releases with the same name can differ a lot, and only the official announcement describes a given one.