When you have LW open, the contents of your shared folder(s) will be available to other users. If you see the firewall icon (brick wall in front of the blue globe), other users will have a hard time downloading from you. There's some info here about how LW & the Gnutella network work.
LimeWire - Wikipedia, the free encyclopedia
Gnutella - Wikipedia, the free encyclopedia

People can only download files that you've chosen to share - so make sure that you're not sharing sensitive or private files. Before you start using LW, you should read through the LimeWire FAQ and User Guide