Overview
minecraft:tags is a block component: it goes in the components object of a custom block. Adds tags to the block so tools, recipes and scripts can find it by group, like "minecraft:is_pickaxe_item_destructible". Write tags as "namespace:name".
Example
Inside the components object of BP/blocks/<name>.json (values are the defaults or placeholders):
BP/blocks/<name>.json
{
"minecraft:tags": {}
}