Net carbs are the carbs you have left when you subtract fiber and sugar alcohols from total carb count.
The problem with counting net carbs is that it can lead to a higher total carb intake and a higher sugar intake which could be harmful to ketosis. If you are new to Keto, it is recommended that you stick to tracking total carbs and not net carbs to ensure that you are not consuming too much sugar.
If you have been following Keto for a while, you may be able to transition to tracking net carbs but you should take time to test your tolerance and make sure that you are not over consuming carbs from taking this approach. If you notice a dip in energy levels, increase in hunger, stall in progress, or lower ketone levels; chances are you are overdoing it with your carbs.