Welcome to AA! You came to the right place.
If you've not learned about the nitrogen cycle, I would strongly recommend you read up on tank cycling. What daileene is referring to is ammonia that has been introduced to your tank because of fish and snail waste. This will start your cycling process, which can take up to a month or two.
I strongly recommend getting yourself an Ammonia, Nitrite and Nitrate test kit. This will help you monitor your Nitrogen levels and ensure that none of these are too high. Too high levels will stress (and possibly kill) your fish... :-\ If you're unable to buy one for now, I recommend doing a Partial Water Change (PWC) twice a week. Just siphon out a gallon of the water in your tank, dump it in the sink, fill it with new water, treat with a dechlorinator (yes, you need to treat the water before putting it in your tank), and add it back into the current tank water.
Also, a common mistake is to change the ENTIRE tank's water. Don't do that or you might find your betta floating belly up. :-\ We're here to help step by step, so please don't be afraid to ask any questions that might come up!