What is unsubscribe in Sveletekit?

When I tried to create persited store in SvelteKit, I had to understand unscribe function like below.

<script>
    import { onDestroy } from 'svelte'
    import { writable } from 'svelte/store';

    const count = writable(0, () => {
        console.log('Start subscribing')
                // Below function is called when execute unsubscribe function.
        return () => console.log('Stop subscribing');
    })

    const increment = () => count.update(value => {
        return value + 1
    })

    const unsubscribe = count.subscribe(value => {
        console.log(value)
    })

        onDestroy(unsubscribe)
</script>

<button on:click={increment}>
    Increment
</button>
<button on:click={unsubscribe}>
    Stop subscribe
</button>

What is unsubscribe function?

If we use subscribe, we should stop the subscribing. Because when component is died, we cannot stop it and use lots of memory. So we should call unsubscribe function with onDestory function. But it is too trouble to develop... ... so we can use $ prefix as instead.