bash interpreter is the default shell on Linux. A shell is the intermediary between the user and the Linux operating system when working at the command line. Shells also can be programmed, much like other interpreted languages such as Python or R. A program for an interpreter is usually called a script.
Scripts are simple ways of bundling up a series of commands to run in order. They can be used to automate repetitive tasks, to run programs in the background, to handle conditions the program may encounter, and so forth.
Anything a user can type on the command line can be part of a shell script.