So basically i wanted to Mask myself using the Pen Tool so i can do some effects on me, I started to connect the points and when i'm done suddenly all the background changes to Black !
could you please help me !
the mask is changing but you don't see the result because the Render is turned OFF. if you want to see the result you need to tick the Render option
now you will see the layer cut on black (which is your background color)
another way to work with masks is to choose one of these modes
this will show you the result on top of the layer itself, or a transparent background, or the composition background color
if you want to see THE RESULT COMPOSITE , you need to click on the composition window
now you will see the layer cut on top of your background
another way to explain:
the Layer window - this is where you can edit your mask and use different ways to see it - but not in the context of your composition. it is a place where you can work on your layer in the context of itself. you can choose to "Render" (tick the "Render") the mask or any effect or not (untick the "Render"). but this is just the layer. it's not your Composition. you can only see the layer cut on itself, on black, on transparent, on red, and a luma matte. that's it.
if you want to see the layer you cut in the context of your composition, you need to get back to your composition window here:
a good practice would be to set BOTH windows side by side and get the best out of both worlds - cut the mask in one window, see the result on top of the background plate in the other.
or like I wrote before, you can choose the other, less complicated, but more limited way of working - in the composition window you set your mask to NONE, do the work and when you are done, set it back to ADD.
see if you get it 
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