Sending stones are large stones (usually around 6ft tall and 3-5 ft in diameter) that are used for near-instant communication across the continent. However, they are very expensive and require regular upkeep and complex rituals to maintain. Not only that, but only the largest and most expensive of Sending Stones can be paired with multiple other Sendibg Stones at once. Most can only support communication with 1, maybe two stones.
As a result it’s usually only large towns that have them, and it’s even more rare for someone to own one all to themselves (usually only royalty or otherwise bigwig types)
What they’re usually used for is sending short messages, usually no longer than a paragraph or so. Usually you send a message to a hub stone, and then wait for that stone to forward your message off to somewhere else. These are usually one-way, though you can require a response for an additional fee.
The various kinds of magic involved in running a Sending Stone is called Telegraphy, with their maintainers called Telegraphs.
They are basically the magic equivalent of telegrams.