What does +1 mean in Starcraft 2?

When ever casters use the term +X/+X They are referring to the weapon upgrade first, then the armor upgrade next. In your example +1 zealots would be a regular zealot with +1 weapon upgrade with nothing else.

You'll often hear them say casters saying "They're going for a quick +1 timing attack" which means that the person researching the weapon upgrade is building an attack for when the weapon upgrade finishes, giving them a timing window/advantage over their opponent (assuming the opponent doesn't have any upgrades).

It appears to mean all of the tier 1 upgrades to weapons, armor and shields. Also seems to potentially include leg upgrades at least in the first game. It would not include air weapons.