What is the backslash(\) used for in SwiftUI?

In SwiftUI, the blackslash operator is used to refer keypath to use inside given block.

from apple:

Add the ability to reference the identity key path, which refers to the entire input value it is applied to.

So for example, see this code:

ForEach(["iPhone SE", "iPhone XS Max"].identified(by: \.self)) { deviceName in
        LandmarkList()
            .previewDevice(PreviewDevice(rawValue: deviceName))
}

here while iterating through array, use the self(here - string) as key

Now take another example: where we use array of objects(not string), now in that case the key which is used as key inside block for iterating is id.

List(landmarkData.identified(by: \.id)) { landmark in
        LandmarkRow(landmark: landmark)
}

\.self it's a identity keypath that apple added for:

Add the ability to reference the identity key path, which refers to the entire input value it is applied to.

More info in proposal.