Bash is the most standard shell on Unix systems (Linux, MacOS, ...). A shell is a basic programming language to interact with the computer and launch commands.
There are standard commands (corresponding to programs) to do many basic tasks. You can read this short document https://files.fosswire.com/2007/08/fwunixref.pdf.
For example to get the current working directory, one can use:
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``` bash
pwd
```
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We see that we ask `pwd` and we get an answer (as a string).
There are also commands that do something but does not returb anything, for example cd (for "change directory"):