What does the question mark in terminal command mean?

Generally speaking, in Bash, a ? is a glob pattern that expands to an arbitrary character.

For example:

$ echo Hello1 > foo1
$ echo Hello2 > foo2
$ cat foo?
Hello1
Hello2

It is akin to a *, but a * expands to 0 or more characters, while a ? expands to exactly one (arbitrary) character.

In your special case though, the ? in the command was apparently a typo.

Those are called Wildcards (globbing patterns)

Standard wildcards (also known as globbing patterns) are used by various command-line utilities to work with multiple files.
Standard wildcards are used by nearly any command (including mv, cp, rm and many others).

• (question mark)

  this can represent any single character. If you specified something at the command line like "hd?" GNU/Linux would look for hda, hdb, hdc and every other letter/number between a-z, 0-9.

• *(asterisk)

  this can represent any number of characters (including zero, in other words, zero or more characters). If you specified a "cd*" it would use "cda", "cdrom", "cdrecord" and anything that starts with “cd” also including “cd” itself. "m*l" could by mill, mull, ml, and anything that starts with an m and ends with an l.

• [ ] (square brackets)

  specifies a range. If you did m[a,o,u]m it can become: mam, mum, mom if you did: m[a-d]m it can become anything that starts and ends with m and has any character a to d inbetween. For example, these would work: mam, mbm, mcm, mdm. This kind of wildcard specifies an “or” relationship (you only need one to match).

• { } (curly brackets)

  terms are separated by commas and each term must be the name of something or a wildcard. This wildcard will copy anything that matches either wildcard(s), or exact name(s) (an “or” relationship, one or the other).

---

For example, this would be valid:

• cp {.doc,.pdf} ~

  This will copy anything ending with .doc or .pdf to the users home directory. Note that spaces are not allowed after the commas (or anywhere else).

• [!]

  This construct is similar to the [ ] construct, except rather than matching any characters inside the brackets, it'll match any character, as long as it is not listed between the [ and ]. This is a logical NOT. For example rm myfile[!9] will remove all myfiles* (ie. myfiles1, myfiles2 etc) but won't remove a file with the number 9 anywhere within it's name.

• \ (backslash)

  is used as an "escape" character, i.e. to protect a subsequent special character. Thus, "\” searches for a backslash. Note you may need to use quotation marks and backslash(es).

for more examples: visit this page