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) {
 
    // default white text
    tooltip.add( new TranslatableText("item.tutorial.fabric_item.tooltip") );
 
    // formatted red text
    tooltip.add( new TranslatableText("item.tutorial.fabric_item.tooltip").formatted(Formatting.RED) );
}

You can also append your tooltip in the block class by overriding a similar method :

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