Well, if you look up the dictionary, the first meaning of stock is the supply of things you keep and can use when you need them.
如果查字典的话呢,stock的意思就是(商店的)现货,存货,库存

But here, to take stock of sth is you to think carefully about it, before taking action accordingly.
而take stock of sth的意思,就是对某情况加以总结,作出评估,进行反思

It was time to stand back and take stock of his career.
是时候停下来对自己的职业进行下反思了.