Hybrid strings are where the racket is strung with two completely different strings in the vertical mains and the horizontal crosses. This can be as simple as using different gauges (thickness) of the same string but more often, different types of string are used; for example, a monofilament string in the mains and a multifilament string in the crosses. Hybrid strings allow the player huge flexibility when deciding on a string setup. Combining different strings can offer a better balance of the different characteristics of their chosen strings. Some of the more common reasons for choosing hybrid strings is to improve comfort or add durability.

There are pre-packaged strings that you can buy of more popular hybrid choices. For example, Luxilon Roland Garros Duo Power Tennis String 16G (12m set) which features Alu Power and NXT Power to create the ultimate power string.
For more flexibility, you can pick from individual string sets and combine them together to create the hybrid string most suitable for you.

Hybrid pros:

Offers huge flexibility to the player to balance certain qualities of strings, or enhance them, to best suit their playing style.

Hybrid cons:

Finding the perfect combination of strings to suit a playing style may take time and be trial and error.