If your child is crying because they're scared or upset about something then it's okay to comfort them.
But if they're crying because of a tantrum because they can't get or do what they want to do then you should not comfort them and give in to them.
If you give into a child when they have a tantrum over something then they'll find out that if they just throw a tantrum and not give up then the parent will give in if they just stick with throwing a tantrum and crying.
Giving into your child when they have a tantrum over something such as a toy you can't get them or something they can't do is the worst thing you can do for a child.
And you'll regret giving into them when they get to the teen years.