Why is there ice on my air conditioner lines?

if it is on the larger copper line than the blower motor is not operating while the condenser fan and compressor are = bad news for compressor if this keeps happening

the system may be low on refrigerant due to a leak. This will still have air blowing out at the registers.

last and least likely is a restriction in the copper lines or part in the refrigerant system. As the refrigerant passes through the restriction the pressure and temp drop and you see ice. Could be on the liquid line.