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In Bash, `IFS` is an internal variable and it stands for ["Internal Field Separator"](https://tldp.org/LDP/abs/html/internalvariables.html#IFSREF) <- according to this link, it "_determines how Bash recognizes fields, or word boundaries, when it interprets character strings_".

Its default value is a "[whitespace](https://tldp.org/LDP/abs/html/special-chars.html#WHITESPACEREF)" (space, tab, and newline), but you can change it to whatever you need.

To give an example, using the default value, the following commands:

```bash
text="a:b c-d e/f"
for word in $text; do echo "Word: $word"; done
```

would ouput:

```none
Word: a:b
Word: c-d
Word: e/f
```

Note that the spaces were used to split the string into fields/"words", so each iteration of the `for` loop gets one part of the split results.

But if we change `IFS`:

```bash
IFS=':-/'
text="a:b c-d e/f"
for word in $text; do echo "Word: $word"; done
```

Now the output is:

```none
Word: a
Word: b c
Word: d e
Word: f
```

By setting `IFS=':-/'`, I'm saying that `:`, `-` and `/` should be the characters used to determine a field/"word" boundaries, thus the result is quite different (note that the spaces were "ignored", so "b c" and "d e" are considered two fields/"words").

If we change to `IFS=':'`, only the `:` character will be considered, and the result would be only 2 fields: "a" and "b c-d e/f".

---
`IFS` is used by other commands, such as `read`:

```
IFS=':'
echo "abc:def" | (read x y; echo "x=$x y=$y")
# output is "x=abc y=def"
```

And it also affects the output of [the special variable `$*`](https://bash.cyberciti.biz/guide/$*) (which contains all the command line arguments of a script), when printed inside double quotes. Suppose I have this simple script:

```bash
#!/bin/bash
echo "Args: $*"
```

If I run this script: `script.sh a b c`, the output will be `Args: a b c`.  
But if I change it to:

```
#!/bin/bash
IFS=':'
echo "Args: $*"
```

The first character of `IFS` will be used in the output, and displayed between the fields, so the output will be: `Args: a:b:c`.


---

One detail regarding whitespace versus non-whitespace characters: if `IFS` contains whitespace, a sequence of one or more whitespaces is considered to be a single separator, but a sequence of one or more non-whitespaces isn't. Example:

```
# text with 4 spaces before "c", and a trailing space in the end
text='a::b    c '
# IFS is just a space
IFS=' '
for word in $text; do echo "Word: [$word]"; done
```

In this case, `IFS` is just a space, but a sequence of one or more spaces is considered to be a single separator, so the output is:

```none
Word: [a::b]
Word: [c]
```

If we set `IFS=':'`, now the field separator is a non-whitespace, so a sequence of one or more is not considered a single separator, and the output would change to:

```none
Word: [a]
Word: []
Word: [b    c ]
```

The second field is an empty string, because each `:` is another separator, and `::` is considered "an empty string between two `:`".

---
But anyway, the field separator can be whatever you need, not limited to the ones defined in the question.