The te form of a verb, followed by the verb shimasu ("finish," "put away"), expresses with emphasis the completion of an action and, in some cases, may imply a sense of regret for an action one has taken or an event that has occurred. Shimasu here is used as an auxiliary.
Kare wa ichi-nen de sono shōsetsu o kaite shimatta.
He finished writing that novel in one year.
Takeshi wa kukkī o minna tabete shimaimashita.
Takashi ate up all the cookies.
Sukkari shukudai o wasurete shimatta.
I have completely forgotten (to do) my homework.
Hope this helps!
°˖ ✧◝(○ ヮ ○)◜✧˖ °こゆうきあいはら°˖ ✧◝(○ ヮ ○)◜✧˖ °