What to wear to a casual wedding?

I’ve been invited to a wedding, last minute, for my fiancé’s grandmother. I asked him what should I wear and he said “something casual”.

It’s very soon on May 16th, it’ll be outdoors in their backyard by a canal at like 6pm. Ive only been to three weddings in my life and each time I was required to dress up. I’m not sure what is considered “casual” to a wedding. My fiancé said he is just wearing slacks and a nice shirt.

Please help me 🤦🏻‍♀️

I was thinking about ordering something like this(I live in Louisiana it’s very hot and humid down here) so I wouldn’t sweat to death.