Hypertrophy or increasing muscle mass is the use of progressive resistance exercise to control and develop musculature. So lifting weights in a certain set and rep range, targeting individual muscles will create an increase in muscle mass.
Alongside training, a lot of it comes down to calories. Here's a great article with a lot of detail about how to combine strength training with a diet that will build muscle.
Our programmes aren't focused towards weight gain/increasing muscle mass, however it's often a by-product of following a programme, and a lot of our clients experience a change in body composition.
Combining this with a good diet will help you gain some weight. Check out this article which explains our approach to diet and nutrition. This has some helpful insights into good ways to eat. Just keep it simple, make sure you eat good quality food, and listen to your body. This will fuel your training and subsequently allow you to develop improved aesthetics.