A piece of (open source) software is a program, an application, or a server [proapse], only if you can start its binary form with your normal program launcher, or (in case of a text file which still must be interpreted by an interpreter like php, perl, bash etc.) if you can start an interpreter which takes the file as one of its arguments and executes the commands. In all other cases, it is a snippet, a module, a library or a plugin (snimoli)
You are using a piece of open source software as an embedded component of a larger unit if [a] one of your files of the larger unit contains a verbatim or a modified copy (i.e. a snippet) of the received open source software, or [b] if your larger unit contains an include statement referring to a functionally defining file of the received open source software, or [c] if your larger unit calls a function defined in the received open source software, or [d] if your development environment contains a compiler or linker directive referring to the received open source software (binaries) and if your larger unit can't be executed without resolving this linker directive.
On the opposite, if you are using an open source application [proapse] (which can directly be executed by your operating system or by an interpreter) or if you are distributing an open source snippet, module or library without embedding it into a larger 'on-top application', then you are using this piece of open source software independendly.
So, we can say that ...
You are modifying a piece of (open source) software if you expand, reduce or modify at least one of the received software files, and - in case of dealing with binary object code - if you (re)compile and (re)link the modified software to a new binary file [modified]. But if you use / distribute the software as you have got it, you do not modify it [unmodified].
Note: if you only modify some of the configuration files, you are not modifying the open source software itself.
You are using the received open source software only for yourself (4yourself), if you as a person do not pass it to other entities like persons, organizations, companies etc., or if you - as a member of a specific development group - pass it only to the other members of your development group. But if you store open source software on any device such as a mobile phone, an USB stick, etc. or if you attach it to any transport medium like email etc. and if you then sell, give away, or simply send this device or transport medium to anyone (other than a direct member of your development group) then you indeed handover the open source software to third parties (2others).
Sometimes it is up to you to decide whether you want to distribute only the binaries or whether you are intentionally going to distribute the sources (too). These are the two forms of distributed open source software. But in some cases, you have to respect some special conditions if you want to distribute the sources and/or the binaries.