It's like ? in many other regular expression engines, and means "match zero or one of whatever came before it".
In your example, the \? is applied to the [ -], meaning it tries to match a space or a minus, but that the space or minus is optional.
So any of these will match:
555 1234
555-1234
5551234
The reason it's written as \? rather than ? is for backwards compatibility.
The original version of grep used a different type of regular expression called a "basic regular expression" where ? just meant a literal question mark.
So that GNU grep could have the zero or one functionality, they added it, but had to use the \? syntax so that scripts that used ? still worked as expected.
Note that grep has an -E option which makes it use the more common type of regular expression, called "extended regular expressions".
man 1 grep:
-E, --extended-regexp
Interpret PATTERN as an extended regular expression
(ERE, see below). (-E is specified by POSIX.)
-G, --basic-regexp
Interpret PATTERN as a basic regular expression (BRE, see below).
This is the default.
...
Repetition
A regular expression may be followed by one of several repetition operators:
? The preceding item is optional and matched at most once.
...
grep understands three different versions of regular expression syntax:
“basic,” “extended” and “perl.”
...
Basic vs Extended Regular Expressions
In basic regular expressions the meta-characters ?, +, {, |, (, and )
lose their special meaning; instead use the backslashed versions
\?, \+, \{, \|, \(, and \).
Further info:
