Hidden altars are places where you can trigger small environmental events like fish jumping out of the water or frogs appearing out of nowhere etc.
Usually all of these have a sign with a bowing person before them. Swipe down on the touchpad to make your character bow and the event will be triggered.
There are 25 hidden altars around the map, but you only need 10 of them. Some will trigger the same effect but all of them count.
Use this map to find all the locations easily = https://mapgenie.io/ghost-of-tsushima