The mask is the treatment that takes your routine to the next level: compared to conditioner, it has a richer texture and a more intense action, and should be left on for a few minutes after shampooing. In this broad category, you will find masks for every need, from nourishing ones for dry hair to restructuring ones for damaged lengths, as well as formulas for color-treated or blonde hair.
To choose, start with your main concern: if your hair is rough and dehydrated, opt for a moisturizing mask like Olaplex's Rich Hydration Mask; if it is weakened by bleaching and flat irons, try a restructuring one, for example, Fanola's Restructuring Mask. One or two applications a week is a good starting point.