Hi Gail, ideally covers should be breathable so you don't get a build-up of condensation which, in extreme cases, can result in the aluminium frame becoming oxidised and discoloured.
For short periods though, a plastic cover is fine for protecting your furniture from rainfall whilst exposed to the elements. For long term storage though I would suggest a canvas style fabric such as AeroCover (https://aerocover.com/), for example.
Most outdoor furniture cover specialists offer a breathable option in a variety of generic sizes. Failing this, covers with air vents are also readily available.