Can anyone answer my simple questions?

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Hi! I’m really new to flash, and I’m not understanding it very well. I just need someone to help me with some real basic stuff. For example, what do the little symbols in the timeline mean? The squares and circles…I was just working on a cartoon, and had just selected something I wanted to turn into a symbol, but I accidently pressed F7, and now all the frames between the one I was working on and the last one have little white circles in the timeline, and are blank when I try to play the video! How do I get the figures to show up again, and what do the open circles mean?