preCICE is a generic coupling software that allows to couple existing simulation codes with each other. The software follows a peer-to-peer library approach. This means that a coupled simulation code calls the coupling software (not the other way around) and that no additional server-like coupling executable is required. The main research goal of preCICE is to minimize the time required to prepare an existing simulation code for coupling. To this end, preCICE regards coupled simulation codes as black boxes without access to internal numerical details. Moreover, preCICE uses a high-level API with only a single "do-the-coupling" call per timestep, no explicit "send/receive" routines. In recent years, preCICE has become a large community project with many active developers, users in various application areas, and a extensive ecosystem of adapters and tools around it. In this presentation, I focus on the fundamental software concepts of preCICE, besides an overview of high-performance computing and numerical aspects.