what does Tilde image mean in MATLAB?

I am working on a code for image processing in Matlab and the thinning won't work unless I call the function on the original image with the tilde and then save it to the same variable (found it somewhere on the internet).

 I= bwmorph(~I, 'thin', inf);
 I=~I;

My question is, what does the tilde do/mean here?