what is the word " ain't " mean ?

make a memory i'm not too old to write about memories, but yesterday when i was walking with my friends i felt that i wanna to keep that memory forever and never get old . i was so happy and grateful for that moment . at that time , i felt that i lost lots of happy memories by being sad and angry about bad memories and forget that i had a very happy memories too . always say to myself live the moment happy or sad because all of that memories make me who i am now .