If someone says this, they want to hit your open hand against theirs as a way of congratulation or greeting.  Give me five可不是“给我五块钱”，其实give me five在运动比赛中很常见，是球队队员中的激励。除了give me five，还有一个give me a high-five。所不同的是give me five是双手平伸而击掌，而give me a high-five是高举双手而击掌。

Examples:
He always gives me a high-five when I win the game. 当我比赛获胜的时候，他通常与我双手高举击掌。
The player said to his partner, "Give me five!" 参赛者对身边的伙伴说：“来，击个掌吧!”