What is it and where is it? Can we get a closer picture?

Even evergreens do shed leaves or needles, they just don't go bare and dormant, but I can't tell what's going on, as we're too far away to see any detail.

Thuja occidentalis cultivar such as 'Nigra'. All plant leaves have a life span; your arborvitae is shedding old interior foliage sprays. As it will do every summer or fall. This is not a problem unless it appears it is dropping more of them than normal. Such as it might do if stressed by drought.

you are really testing old eyes ..

interior yellowing.. then browning is normal.. cyclical ... and increased by weather.. like bad winter prior of drought in summer .. check out the link below

get a pic of two closer to confirm such ...

ken

https://www.houzz.com/discussions/4829252/interior-yellowing-is-normal#n=6

Checked for bag worms?