A C# Minor (C#m) chord contains the notes C#, G#, E. It is built from the minor formula 1 - ♭3 - 5, with C# as the root.
Why is the low E string muted in C#m?
The open low E isn't in a C# minor chord, so letting it ring would blur the harmony. The barring index finger lightly damps it, which is why this voicing starts on the A string rather than the sixth.