The lower back is trained indirectly in basically every core or compound exercise, such as squats, deadlifts, pullups, rows etc. The lower back helps keep your spine safe and your body rigid. This will allow you to lift heavier weights much safer. To isolate the lower back (for adding extra muscle and strength to the area, for injury prevention/aesthetics), Glute Ham Raises, Reverse Hyper Extensions, Supermans and Bird Dogs are some great starting exercises. If you are already doing the basic compound lifts (squats, deadlifts etc), pick one lower back isolation exercise, for 3 sets of 15-20 reps at the end of your workout. Test it and see how that improves your compound lift performance/muscle building goals!