Tinker - it means to manipulate small things, especially in a mechanical or electrical device, to improve it or just to see what happens. Example:
Your computer doesn't work right, so you "tinker" with the settings. You change the monitor, the speed, and so on, until you see if you can get it right.
Another example:
He tinkered with his watch. He opened it up and moved the battery around. :) something like that!