Aameen means 'May Allaah it happens like that (what is being asked in dua).
Aameen is to be said by those who listen a dua being asked by other than himself. For example if I ask a dua " May Allaah bless you" then the person who listens this dua may say Aameen as well as Summa Aameen.
The person who himself ask any dua is not required to say Aameen at end of dua.
Assalaamu'Alaikum WarahmatUllahe Wabarakatuho
The Meaning of Aameen is as my brother said in above answer. And about its importance please read the following Hadith:
"(1) Narrated AbuZuhayr an-Numayri: AbuMisbah al-Muqra'i said: We used to sit in the company of AbuZuhayr an-Numayri. He was a companion of the Prophet (peace_be_upon_him), and he used to narrate good traditions. Once a man from among us made a supplication. He said: End it with the utterance of Amin, for Amin is like a seal on the book. AbuZuhayr said: I shall tell you about that. We went out with the Apostle of Allah (peace_be_upon_him) one night and came upon a man who made supplication with persistence. The Prophet (peace_be_upon_him) waited to hear him. The Prophet (peace_be_upon_him) said: He will have done something which guarantees (Paradise for him) if he puts a seal to it. One of the people asked: What should he use as a seal? He replied: Amin, for if he ends it with Amin, he will do something which guarantees (Paradise for him). Then the man who questioned the Prophet (peace_be_upon_him) came to the man who was supplicating, and said to him: So-and-so, end it with Amin and receive the good news."
PS: There is a hadith which can be translated some how as: Indeed the prayers (supplications) are between earth and sky. Part of it not goes up, unless you send Durood/salat on your Messenger. (Mudalul Namaz, section Dua with reference to Thirmidhi) - Due to this reason many Scholars recommends to say Salawat/Salat ala Nabi/Salat un Nabi/Darood/Durood before and after making supplication/Dua.
AND ALLAH KNOWS BEST