Let's say a company had a plan where there is zero vesting until three years, and then it's 25% until six years. Now let's say that they have a profit-sharing account and a matching-fund retirement account. The first only the company contributes to, and the second, they match whatever you put in at 25%.
Yes, I'm sure 'vesting' can become complicated. However, my experience of this has always been simple. I've only encountered this in connection with the topic of pensions.
Tom works for a company for ten years, and the company holds money that will be used to pay Tom a pension at age 65. Tom is 40. He leaves the company. Depending on various factors, he can either take
(I say that as a joke, of course, but in the U.S., the traditional pension has pretty much disappeared and has been replaced by employee funded retirement accounts. If there are employer matches, that's when the vesting comes in.)