The most common use of "casting" probably isn't what you think. That is, it doesn't really send the content from the mobile device to your casting device.
App CastingCommonly, someone would launch an app on a phone or table -- say, Netflix -- and select a video to play. The user would press the "casting" icon on the screen and select the device to play the content.
Screen MirroringSome people confuse app casting with "screen mirroring." Apple devices use a process called "AirPlay" to accomplish actual screen mirroring. Miracast is another process supported by some Android and Microsoft devices. AirPlay is generally more reliable, but some users have had great success with Miracast.
To mirror your screen, you would use the appropriate command to open the option -- since it's device-specific, not application specific, it would vary from device to device -- and select your mirror device from those found on the network.