This is the 5667th most frequent Japanese word.
すすん
"すすん" is the continuative/conjunctive form of the verb "進む" (susumu), meaning "to advance" or "to progress."
Here, 'すすんで' reflects progress or movement, indicating the road has been traversed towards a destination.
道がすすんで、街に到着しました。
The road advanced, and we arrived in the city.
In this sentence, 'すすんでいる' describes the development or advancement of someone's thoughts or ideas.
彼の考えはいつもすすんでいる。
His thinking is always advanced.
The word 'すすんでいる' here is used to imply ongoing progress in the development or improvement of technology.