Real estate websites like Zillow use statistics and data from public records to estimate home values. They do have a lot of information, and can provide a lot of helpful information as well; they are good at giving you a broad estimate of your home's value, and offer good indicators as to how the market is performing. However, the inaccuracies in their system often mis-value homes as worth much more or much less than they really are.
What are some of the downfalls? Well, for starters, they aren't looking at current hyper-local conditions. In Atlanta right now, we are sitting at around 3.7 months of inventory, putting us in a hot seller's market. However, saying that is like saying it's 70degrees in the United States. Each neighborhood is different. School districts, price range, and location all play a factor in a home's value. Zillow doesn't take this into account, nor do they take into account any improvements or renovations you have done to your house that may have increased its value by thousands.
If you want to know your home's true value, you should call a local agent who is familiar with the local area, like myself. I also have 9 years of real estate appraisal experience and have worked in the area for over 10 years.