In your item class, override appendTooltip like so (see lang for how to translate the tooltip) :

@Override
public void appendTooltip(ItemStack itemStack, World world, List<Text> tooltip, TooltipContext tooltipContext) {
    tooltip.add(new TranslatableText("item.tutorial.fabric_item.tooltip"));
}

For a block, do the same but override buildTooltip in your block class.