Linearization of a PDE

Dear all, why the linearizaiton of the following PDE
$(1+u_y^2)u_{xx}-2u_xu_yu_{xy}+(1+u_x^2)u_{yy}=0$
is
$u_{xx}+u_{yy}=0?$
Thank you very much.

Indeed, I do not know what linearization means. Thank you in advance.