This is the 15618th most frequent Russian word.
приехавший
"One who has arrived."
The word 'приехавшего' is a past participle modifying 'друга' (friend), indicating that the friend has arrived recently.
На вокзале мы встретили недавно приехавшего друга.
At the station, we met a recently arrived friend.
Here, 'приехавший' is a past participle acting as an adjective describing 'автобус' (bus), signifying the bus has recently arrived.
Приехавший автобус оказался переполненным.
The arriving bus turned out to be full.
In this example, 'приехавшим' is the instrumental form of the past participle, referring to the professor as someone who has arrived.