I have a couple of these. They can hold different things, commonly carbon filtering from coconut husks. Also a “template” water softener which would look like small plastic beads inside. But this Looks like this is just part of a complete system. If it were a salt water softener there would be a control head somewhere and brine tank to add salt without undoing any plumbing. It could be a template water softener which just forces hard water through these beads and supposedly changes the calcium structure making it appear soft. But it might also have another canister like that for carbon filtering. something like this