Zenith means the high point — it comes from astronomy, where it describes the highest point in an arc traveled by a star or a planet or another celestial body. The sun reaches its zenith when it is as high in the sky as it is going to go on that day.
Zenith — and its opposite, nadir — have been appropriated by non-astronomers and are used to describe high and low points in non-astronomical contexts. For example, you can say that the high school quarterback hit his zenith in high school, and it was all downhill from there. Do yourself a favor and try to delay your zenith so you'll have something to look forward to in your future!