Toddlers and preschoolers are anxious to learn to do things for themselves but often need some guidance. This need for help along with the fact that they haven't mastered emotional regulation yet leads to frustration and meltdowns. Sometimes it's all a parent can do not to have a meltdown right along side them!
Find ways to encourage their budding independence by giving them as many tasks they can successfully handle. This is the perfect age to start letting them help out with simple chores. They love to help and practice new skills ... and soon enough they'll be old enough to handle these chores on their own.
For more ideas on parenting young children, check out the online parenting class for ages 5 and under.