Carabiners are strong metal loops with a spring-loaded gate. They are used for climbing, in safety-critical systems, and for securing items to your pack. Your pack should include at least one carabiner rated for climbing. Carabiners are used for a variety of purposes and come in a variety of shapes and formats.
This is a typical D-style carabiner. The numbers on the spine indicate the weight capacity in kiloNewtons. Only use carabiners that have a kN rating engraved on the spine. A kiloNewton is a force of gravity rating (not static weight) equivalent to 225 pounds (kN to Pound-Force Converter). Force equals mass times acceleration and these devices must absorb the force generated by a fall. You should also check and/or replace worn gear. The ratings are for the strength along the spine, the gate, and along the spine with the gate open. In this example the values are 24, 7, and 7 which means that this carabiner can hold 5,400 pounds along the spine and 1,575 pounds along the gate or with the gate open.
This is a symmetrical D-style carabiner. The D-style can generally hold more weight because the ropes concentrate the force close to the spine. O-style carabiners generally hold less weight because the ropes are farther from the spine. There are many other styles of carabiners designed for different climbing requirements.
Carabiners are useful for holding items on your pack. Many people use the wimpy version sold in many retail stores. These work for holding small, lightweight items but should NEVER be used for climbing or safety purposes. Climbing grade carabiners can hold your stuff but they can also do real work when required. I recommend the simple D-style carabiner that can hold at least 13 kN. I prefer the type that are smooth all the way around.