Source code contains the instructions on how a program should function, written in a human-readable programming language.
The source code contains the design and building blocks that a program is created from. Software developers can modify the source code to fix, modify or enhance the behaviour of a program created from it. For larger programs it is not uncommon for there to be thousands of files of source code, totalling hundreds of thousands or even millions of lines of code.
Almost all programs are distributed in a form that is ready to run. The way source code is converted in to a runnable program can be quite complex, and can depend an the programming language that it has been written in. However, there will commonly be a stage where the program is compiled into object code or byte code. In this form the program is just a long list of numbers and so isn't human-readable.
With software that is proprietary or closed-source, you will never be able to see or change the source code it was created from. The person, team or organisation that created it maintains exclusive control over it. This means that it is impossible for anyone, except the people that created it, to fix bugs in it, enhance or modify the way it works, or check to ensure that it is working the way that is expected.
Open-source software is different. Its authors make its source code available to everyone, so they can view, copy, modify, and share it. This gives people the freedom to fix, change and enhance the program in any way that they want to. It also provides the transparency that allows people to inspect and audit how a program works, so they can be sure that it is not doing anything they didn't expect it to be doing. And because the source code is freely available anyone can then build and run the program on whatever hardware they like.