From the ELF(5) man pages:
Relocation is the process of connecting symbolic references with symbolic definitions. Relocatable files must have information that describes how to modify their section contents, thus allowing executable and shared object files to hold the right information for a process's program image. Relocation entries are these data.
The process of relocation relies on symbols and sections, which is why we covered symbols and sections first. In relocations, there are relocation records, which essentially contain information about how to patch the code related to a given symbol. Relocations are literally a mechanism for binary patching and even hot-patching in memory when the dynamic linker is involved. The linker ...