A webmix is locked (padlock symbol) when you add a publicly shared webmix to your Symbaloo and receive the updates from the original creator. By clicking Stop updates in the Share menu you can "unlock" the webmix and then edit and/or share it yourself. Note: You will no longer receive the updates.
You are not able to lock your own webmixes with the free version. With a Symbaloo PRO account you can set various restrictions and privileges for your webmixes, like locking them so that users of your solution can't make any changes.
Have a look at how the control panel for branded solutions works: