According to a couple sites among search results, it began in a specific druidic tradition, but has been adopted rather broadly in the reconstruction.
There's a legend of a deity gifting a bag made of the skin of a crane, full of treasures. These treasures basically represent the Ogham alphabet. That is, the means of passing down wisdom. That said, the concept has become a pouch, purse, or bag used to carry personalized ritual items. The exact items are chosen typically by the individual, and vary widely based on the intent of their use. In that sense, they are similar to many witchcraft practitioners' books of shadows, as they are completely personal in nature.
In stead of continuing to summarize, I will post the two links.
The crane bag is simply a bag we use to hold items we use and spiritual items. Ritual knives, harvesting knives, stones, herbs, oils, ritual items, books, small instruments, pens, journals,etc. The possibilities are endless really. There are certain spiritual items we place in the bag to keepsafe, but the bag can be consecrated and used for anything the Druid needs.