This is the 18432nd most frequent Russian word.
энтузиазмом
"Энтузиазмом" means "enthusiasm" in English.
In this sentence, the word "энтузиазмом" is used as an instrumental noun ("с энтузиазмом"), describing the manner in which the action "приступает к работе" (approaches work) is performed.
Он всегда приступает к работе с большим энтузиазмом.
He always approaches work with great enthusiasm.
Here, "энтузиазмом" is used as a direct object in the instrumental case, illustrating a quality that has a potential effect of inspiring others.
Её энтузиазмом можно вдохновить любого человека.
Her enthusiasm can inspire anyone.
In this context, "энтузиазмом" is used as the object of the verb "делится" (shares), demonstrating the thing that is being shared.