Yes, you are correc that (m/w) means male/female or better männlich/weiblich.
The reason it is added is due to German antidiscrimination laws and corresponding court rulings. German has a generic masculine gender for many nouns including most professions. But it also has a specific feminine noun that can be systematically derived from the masculine one by adding -in.
Court rulings have decided that using only the generic masculine noun is discriminatory, since it would imply searching only for male applicants. Instead, it must be made clear that there is no preference for either sex when recruiting. This can be established in the following ways:
Softwareentwickler oder Softwareentwicklerin
The first option is way too long if you consider newspaper ads where every millimetre of space costs money; the same thing goes for the second. The third and fourth are often seen as badly typeset. This also goes for the fifth, but additionally, depending on the typefaces used, it may imply only females are supposed to apply. Finally, the sixth is both unambiguous and short (even though a few characters longer than the third and fourth).
Of course, a variety of other methods are possible as mentioned in the comments, e.g.
Wir möchten eine Stelle in der Softwareentwicklung zum 1.12. neu besetzen.
It is not part of the question whether this assumption made by court rulings is backed by language usage and understanding (by non-feminists, that is). Hence I will not go into those details.