If I were to make an animated film using open source software, legally, what does that mean for me?

There is no money involved in the use of Open Source software is just about all situations and what you created as long as it doesn't involve any other source than your mind is legally your own animation and whether you give it away or sell it that is your option. But as stated read the license agreement of the open source program you will be using. You may be required to add credits to your animation at the end of it.

Read the terms of the open source software for most of your answers. In most cases, anything you make is yours no matter what and you can definitely make money off of it. If the programs doesn't place a watermark on it, then you usually don't have to tell anyone what you made it with (although it's cool to tell people that you made a nice looking film/animation with a free product as opposed to a paid-for product). Sometimes, if you would be selling the product for profit, it would be considered "business use" and you would have to pay for a business license version of the product. Find out if one exists and read the features/specs/etc. It usually states what the purpose of a business license is for. Without knowing exactly which software you are referring to, I can't tell you much more info.