This, quite simply, will be the most reliable and versatile day-in, day-out lens you can put on the front of your Canon DSLR. Why do we say that? Well, first of all, it's in the super-useful "standard" zoom range: 24-70mm for full-frame sensors, and 38.4-112mm on APS-C sensors, so you can get beautiful landscapes and intimate portraits within seconds of each other.
It's also a Canon "L-Series" lens, their premium designation. This means it has superb optics, ultra-quiet mechanical elements, and superior dust and water resistance. And with its maximum aperture of f/2.8, you'll have great light sensitivity to go with the subject-flattering shallow depth-of-field it can generate. If you don't know what photo opportunities await you, this lens is the best place to start your preparations.