The term IBD or Identity by descent is used for DNA segments shared between a minimum of two or more people if those have inherited it from a common ancestor without recombination.
Let's examine a real-life example from one of my triangulated groups:
Simone, Marco, Robert, and I have all inherited a DNA segment on chromosome 2 between 206 mBp (million base pairs) and 217 mBp. This refers to the orange part inherited by all of us.
If we look just at the parts inherited between two people, for example, the IBD segment between Simone and me, you can see that the IBD segment between us is bigger and starts at 206.7 mBp and ends at 216.9 mBp.
IBD segments are important because they help us understand how two DNA tested people are related.
There is a misconception that IBD segments have to be a certain length, based on centiMorgans. Discussions are usually brought up on the minimum thresholds that the DNA testing companies consider (eg. 5 centiMorgan vs 7 centiMorgan).
DNA can be inherited intact without genetic recombination as long as it came from the same ancestor. Professor Itsik Pe'er explains this process in detail in his video Identity by descent in medical and population genomics.