This is the 4th most frequent Romanian word.
cu
"Cu" in Romanian means "with" in English.
In this sentence, 'cu' expresses the means of transportation, indicating the tool or method by which the action of leaving was performed.
Am plecat cu trenul la București.
I left with the train to Bucharest.
Here, 'cu' denotes companionship, showing that the action of going is done in the company of others.
Merg la cinema cu prietenii mei.
I am going to the cinema with my friends.
In this example, 'cu' indicates the tool or instrument used to perform the action of writing.