In short, it's gonna be osu!'s future game client with a brand new look and will be open source (for example anyone can make a new game mode). It's still incomplete and buggy so don't except much from it at the moment.
To add songs to osu!lazer, you gotta pick an osu!beatmap file freshly downloaded from the site and drag it into the lazer client. This is the only way. Only osu!standard is supported for the moment.