Two things here. First the easy one:
“individual” is just another word for “personal”, meaning (in this case) file from your user-account’s home directory. Note in fact the
~/ prefix to all individual/personal files.
On desktop systems like your computer at home you usually have only one user account, but on server computers there can be several accounts registered, and each one has its own home directory and therefore its own personal/individual
Now the hard one, and to try and explain this I need a small preface:
Bash differentiates between “login” interactive-shells and "other" interactive-shells. (I have no experience about other programs like
ksh but I suppose they do too).
For example, on Linux systems a login shell is typically only the very first one that it’s started after you have entered (correctly) your username and password either from textual console or from a network connection such as ssh.
On the contrary, “other” interactive-shells are typically the ones started by a graphic desktop when you click the icon of the terminal emulator application.
“Other” (ie non-login) interactive-shells are also when you subsequently invoke nested interactive-shells even from a login one.
A practical example of this latter case. Suppose you are at the prompt of the first shell after having authenticated yourself (ie logged in) to a remote computer via ssh: that’s the login interactive-shell; but if you then type
bash and press Return you start a new other interactive-shell. You can nest them as many as you like. Only, you need to type
exit (or Control-D) for each of them in order to unwind the stack of nested interactive-shells you started.
Bash’s documentation there tells you which files are automatically executed on startup of login shells and which other files on startup of other shells. There are also a couple of files that are automatically run only when you quit login shells, but not when you quit other shells.