哪位大虾帮忙解释一下关于随机漫步的问题

一篇论文在讨论随机漫步理论的章节中提到： Prices may exhibit trends over time, in order that the total return on a financial asset exceeds the return on a risk-free asset by an amount commensurate with the level of risk undertaken in holding it.

不理解这句话的意思，为什么从长期来看，股价会存在趋势的可能？