Convert to grey-level - what does it mean?

I have imported video footage, then I tracked plate and imported image, and while I importing image, I see this:
Image: http://f.cl.ly/items/0S2j0B343y3Z450D0O2k/Screen%20Shot%202011-11-10%20at%201.40.36%20AM.png
:confused:

This means you imported a matte clip. Matte Clips have to be in grey scale to work correctly. If you want to import an insert clip, use the insert dropdown instead.

Oh yeah yeah I see now, I was tried to insert AlphaClip.
Okay I gonna fix it now, thanks Martin :rolleyes: