According to the Wikipedia definition, Back-Pressure is resistance or force opposing the desired flow of fluid through pipes. But this definition belongs to fluid dynamics. In the context of software, the definition will be changed to flow of data within software instead of fluid through pipes. So, the definition would be-

Back-Pressure is a resistance or force opposing the desired flow of data through software.

When one component is struggling to keep up, the entire system needs to respond sensibly. It is unacceptable for the component under stress to fail or to drop messages in an uncontrolled fashion. Since it is not easy to handle and can't fail, it should communicate that it is under stress to upstream components and get them to reduce the load. This back-pressure is an important feedback mechanism that facilitates systems to respond to load rather than collapse under such a situation gracefully. The back-pressure may cascade up to the user, at which point responsiveness may degrade. Still, this mechanism will ensure that the system is resilient under load and will provide information that may allow the system to apply other resources to make easy the load by distributing it.

In simple words, we can say that Back-pressure provides the strategies for coping with Observables that produce items more rapidly than their observers consume them.