This is the 5125th most frequent Russian word.
намерено
"Deliberately" or "intentionally."
In this sentence, 'намеренно' is used to imply that the action of leaving the door open was done intentionally, with purpose.
Она намеренно оставила дверь открытой.
She intentionally left the door open.
Here, 'намерены' (conjugated form) reflects the intention or plan to do something in the near future.
Мы намерены посетить выставку завтра.
We intend to visit the exhibition tomorrow.
'Намеренно' in this case emphasizes that the actions undertaken were done on purpose, highlighting their deliberateness.