broad是宽阔的，broaden就是变宽和增长见闻。broaden one's horizons 开眼界；开阔视野；扩大视野；拓宽视野

【例句】
Not only can traveling broaden one's horizons, it can relax one's body and soul as well.
旅行既能开阔人的视野，又能放松人的身心。

Watching documentaries is helpful for you to broaden your horizons.
看纪录片可以帮助你开阔眼界。