Word Strength shows the score of your presently played word relative to the maximum possible score you could presently make.
To activate Word Strength, toggle the lightning icon below the game tiles.
Then tap on the Activate button.
An animation will appear to confirm the Word Strength activation.
Once activated, you will see a green checkmark to show the strength of your word. Tapping the checkmark opens the tooltip.
When the time expires, you have the option to re-activate the feature by purchasing it using your Coins.