For example, your employer matches "50% up to 6%." This means if you contribute 6% to your 401k per year, your employer will put in 50% of that contribution.
The optimal situation would be to have a company that matches a high percentage of your income (3% or more), match 100%, and have 100% immediate vesting.
There is no such thing as a return on a 401k. A 401k is an account type your put money into that has a special tax law associated with it. The return you earn will be based on how you choose to invest your money.