In addition to the basic information about the order, the LineItems field contains an array of information about each line item in the order. You can reference this data for building templates, branching logic based on package contents, and so on. This is stored in a 0-indexed array, so you can easily access an individual item or loop over it to get the entire contents.
This product data is retrieved from Shopify and stored by Wonderment as the event is generated. As such, the events regarding a specific fulfillment will reflect the product's data at the time of order. Updates to product catalog data will be reflected in future fulfillments, but not change the data for in-progress fulfillments. This data reflects contents of a specific fulfillment, not order contents, so items not in this fulfillment (ie in another package from a different fulfillment center), or that do not require fulfillment (such as digital gift cards, ebooks, etc) will not appear in this listing.