When you close a mask path the default is to create an alpha channel all around the outside of the closed path. You can set the mask mode to subtract or invert it to reveal everything inside the mask or you can set the mask mode to several other modes.
Each mask also has an opacity setting which you can adjust.
Probably the easiest thing to do when drawing complex masks is to set up another layer that is a contrasting color to the majority of the background, set the blend mode to add or multiply so you can see through the layer, then draw your masks on that layer. This way the colored sections of the layer will be transparent when you use your new layer as a track matte.
To learn about masking, and you should take some time to do so, please type Masking in the Search Help field at the top right corner of AE and look at the help files and the community resources. It's very important that you understand how to use and animate mask paths.