Automatic Logout

Hi I'm a new iMac user i recently purchased one. But i am having a small problem with it everytime i leave for about five minutes it logs me out i dont understand why it does this. This time it made me very mad i was typing a word document and i left just for five minutes when i came back it was logged out and i lost my word domument i was hoping somebody can tell me how to fix this problem.

thank you!

 

Thank you that helps!