What I mean by compound exercises are those that use a lot of muscle groups all at once, for example the squat both engages your legs and your glutes (bum) but also your core (abs and lower back). Just for completeness an exercise that would not count as a compound exercise is a bicep curl. Now there are advantages to both types of exercises.
Firstly, the single group exercises allow you to specifically target that muscle and make it stronger/bigger. It also allows you to vary your workout as there are many more variants of exercises for single muscle groups than there are compound exercises.
When it comes to compound lifts there are three key lifts that you should try; the squat, the deadlift and the bench press. I highly recommend using these 3 exercises as a way of increasing your overall strength as they incorporate a bunch of different muscle groups at one time.
So why is it important to use multiple muscles at once?
Because compound exercises engage many muscles at once they are one of the more time efficient exercises you can do! when you can engage multiple muscle groups at once you can cut down the amount of time you have to spend at the gym, freeing up your day to recover and make the most of that great post-workout feeling! They also reflect what would happen in real life if your fitness and strength was called into question. It also helps to give you a balanced workout, giving you gains all over your body. Nobody wants to look like these guys...