Title is inaccurate; the trials just showed the vaccine was safe for humans, it did not have anything to do with the success of the vaccine in combating HIV infections, that comes in the following testing phases.
Phase 1 just means that it is not very dangerous for humans, it may still have significant long-term side effects and or not achieve a statistically significant improvement in patient outcomes.
To build on this, Phase 1 trials are meant to determine safety - the treatment is given to a smallish group of healthy individuals to look for side-effects and what not. Phase 2 is a small scale test for effectiveness of the treatment and more side effects. Phase 3 is a large scale test of effectiveness and more side effects. After passing Phase 3, the treatment can probably be put out to market, and then 'Phase 4' (post-market) data can be gathered to ensure that nothing was missed.