When you post something on Facebook, you have the possibility to make your post public, to share it with your friends or to keep it for you. The lock icon indicates that the post is visible only for you. In order to change that, you need to press the Only Me button above your post and to select Public, Friend or Custom option. In the Custom section, you have the possibility to make your post visible for specific people. Also, you can set this option for all your posts if you go to Settings > Privacy > Who can see my stuff? > Edit.