One-liner
Lock-in means committing to tie your money up for a set time; optionality means keeping it liquid so you can move when something comes up.
Why it matters
Locked positions tend to pay a bit more, but selling early is not guaranteed at a good price (you are back at ). Liquidity earns less, but it lets you act: in the COVID crash, the people with cash and an appetite for risk got to buy stocks that had fallen hard.
Analogy
A flexible plane ticket versus the low-cost one. You love the cheap fare right up until something changes, a problem or a better opportunity, and you are stuck.
The catch
Treating locked money as if it were available. That is the recipe for handing money back to the market at the worst time and walking away frustrated. Always keep a liquid slice for emergencies and opportunities.