Permissions are assigned to owner, group and others. Depending on who has access to your machine, you would choose the level of security. There is a GUI tool to set permissions, but I know it only from the command line. If the stuff below looks overwhelming, then start by finding the tool in your menus.
First, to see what the current permissions are:
..........You will see a line for every top-level directory. Here is my entry for /usr:
drwxr-xr-x 12 root root 4096 Apr 17 17:57 usr/
Translation: The /usr directory is owned by root and assigned to the root group. The owner has full permissions (rwx), and the group and all other users have read + execute permissions (r-x)
With a directory, you need execute permission to open it, and you need write permission to change anything (including adding a file). Thus, if I am a regular user, I can enter this directory and browse, but I cannot change anything.
Here is one example of how to set permissions:
Suppose I want to create a file in /usr/share. Most things in /usr have the permissions shown above, so first, I will give "other users" write permission to /usr/share:
(to take on root powers)
chmod o+w /usr/share
su username .....go back to my regular username
....creates an empty file "testfile"
As a normal user, I can now create a file in /usr/share (even though I still do not have write privileges to /usr---only /usr/share)
Here's one of many articles on file permissions---found with a Google search: