Tab Completion allows quickly completing words by pressing the tab key. When enabled, pressing tab will expand the text to the left of the cursor into the best match, using Sublime Text's fuzzy matching algorithm.
Tab Completion is enabled by default.
Sometimes tab completion isn't desirable. To turn it off, add this line to Preferences/File Settings - User:
To insert a tab rather than a completion, press shift+tab.
Sometimes the completed text isn't what's desired: to change to an alternative, press ctrl+space. This will roll back the completion, and display the standard auto complete list.
Alternatively, tab can be pressed again to cycle to the next completion.
Information on completion sources, and creating your own completions, is available at sublimetext.info.