When you run your year end, to stop further transactions being posted into the year you're closing, you can apply a lock date. This is useful for preventing late entries once you've finalised your year end figures.
A common question is whether this also prevents genuine previous year postings, for example, if you later receive adjustments from your accountant to make in the previous year. Don't worry, even when you apply a lock date you can still make valid adjustment postings if needed.