How to perform additive blending in Metal. What do these terms mean?
The “blend factors” are the amount that the specified component is multiplied by while blending. “Destination” is the existing content that’s being blended “on top of”, while “source” is the new content that’s being blended “onto” it. When the blend operation is set to “add”, the final pixel value is calculated like this:
value = source * sourceBlendFactor + destination * destinationBlendFactor

…so a blend factor of “one” means the value is used unchanged (multiplied by 1), “[x]Alpha” means the value is multiplied by the alpha value of x (source or destination), and “oneMinus[x]Alpha” means the value is multiplied by (1 - the alpha value of x).
For additive blending, you most likely want the following:
Objective-C:
descriptor.colorAttachments[0].blendingEnabled = YES;
descriptor.colorAttachments[0].rgbBlendOperation = MTLBlendOperationAdd;
descriptor.colorAttachments[0].alphaBlendOperation = MTLBlendOperationAdd;
descriptor.colorAttachments[0].sourceRGBBlendFactor = MTLBlendFactorSourceAlpha;
descriptor.colorAttachments[0].sourceAlphaBlendFactor = MTLBlendFactorOne;
descriptor.colorAttachments[0].destinationRGBBlendFactor = MTLBlendFactorOne;
descriptor.colorAttachments[0].destinationAlphaBlendFactor = MTLBlendFactorOne;

Swift 5:
descriptor.colorAttachments.colorAttachments[0].isBlendingEnabled = true
descriptor.colorAttachments.colorAttachments[0].rgbBlendOperation = .add
descriptor.colorAttachments.colorAttachments[0].alphaBlendOperation = .add
descriptor.colorAttachments.colorAttachments[0].sourceRGBBlendFactor = .sourceAlpha
descriptor.colorAttachments.colorAttachments[0].sourceAlphaBlendFactor = .one
descriptor.colorAttachments.colorAttachments[0].destinationRGBBlendFactor = .one
descriptor.colorAttachments.colorAttachments[0].destinationAlphaBlendFactor = .one

There’s a really helpful tool for visualizing the different options here—it’s written for OpenGL, but the Metal stuff works the same way.