Why my matrix convert to sparse double

my defined variable converted to sparse double after i run the code . what is sparse double and why my variable converted to it .
> NN=10;

cvx_begin quiet
                    cvx_solver Mosek 
                    variable pch(NN,T)
                    variable pdis(NN,T)

finaly p,pch,pdis converted to 10*48 sparse double . how can i fix it

Don’t worry. Just use the sparse variable pch. If you need a full matrix, after cvx_end, you can use
pch = full(pch);

if you really want to understand it, you can study the CVX source code, which is included in the CVX distribution.