Runes are a way to build reactivity in Svelte.
$state
See more on this rune here $state.
$derived
$effect
The effect rune is only executed when being on the client. Therefore, it can be used to run code that should not run on the server. It runs when the component is mounted to the DOM.
$effect.pre
Is run before the component is mounted to the DOM
Runes mode vs. legacy mode
Once components use runes or explicitly state to use the runes mode, they are in runes mode. Alternative is legacy mode.