sdisplay (symbolic display) tries to display an sdpvar object in symbolic MATLAB form. It is primarily used as a debug tool since it can fail in some instances (YALMIP has no internal notion of variable names).
f = [x;x^2+y+x*y];
The command tries to find the symbolic names of variables, but this can fail in some instances, such as when the variable has been defined outside the current scope. Additionally, variables defined using nonlinear operators can currently not be displayed.