This is the 4871st most frequent Russian word.
исполняющим
"Исполняющим" means "performing" or "acting (as)," depending on the context.
In this sentence, 'исполняющим' is used as a participle agreeing with 'он'. It conveys the person temporarily enacting the role.
Он был назначен исполняющим обязанности директора.
He was appointed as the acting director.
Here, 'исполняющим' functions as a gerundive describing performing as an ongoing activity of the composer.
Композитор активно работает, исполняющим симфонии во многих городах.
The composer actively works, performing symphonies in many cities.
In this sentence, 'исполняющим' is used adjectivally to describe something that fulfills or enacts ('dreams' in this case).