This will show how many Builders you currently have available (i.e. those who are not currently engaged in a task), which is crucial if you are trying to work out what tasks to set up in your base - if you are about to go offline for an extended period, for example. While your Builders are busy you are unable even to queue up more jobs for them - the only way is to rush their current job using Gems, or to buy another Builder (again with Gems).
Purchasing more Builders is undoubtedly the best use of Gems in Clash of Clans. They cost a vast amount to purchase, and many players will struggle to get beyond the 2 Builders given to you by the end of the Tutorial. If you can pay for it however, it is certainly worth it, as once you reach higher levels your builders will be engaged in ever-more frequent upgrades that take longer and longer each time. Builders are also needed to remove the obstacles which litter your base when you first begin playing.
You can eventually have a grand total of 5 Builders, if you have the Gems for them.
Each Builder's Hut is illustrated by a smalls hack with a hammer and a saw icon on it
TIP: When placing a Builder's Hut, consider placing it on the edge of the map as this can save you from falling prey to a 3 star victory as the opponent may run out of time or miss seeing it on your map. Similarly, when attacking, keep an eye out for them in enemy bases. Nothing is lost if your Builders' Huts are destroyed while you are defending, so use them also to push the no-spawn area of the enemy armies back further from your center.
Below is a list of each Builder's Hut that you can unlock, and advice on how to do it. Note that 1 Builder is provided by each Builder's Hut:
Cost of Buidlers' Huts
Builder's Hut 1:
This is given to your for free during the tutorial
Builder's Hut 2:
You are provided with the Gems to buy this during the tutorial and it is required that you spend them and construct it before moving on
Builder's Hut 3:
You can either purchase the Gems for this hut, or save hard by completing Achievements and removing obstacles. Note that one level 3 achievement, called Sweet Victory, gives 450 Gems on its own, so this is certainly possible.
Builder's Hut 4:
It is possible to save up the Gems for this, but it's not easy.
Builder's Hut 5:
If you thought the last hut was a stretch to save for, this one is double! You can get a total of 1,412 Gems from Achievements, plus any you saved from the tutorial and those earned from clearing obstacles - technically it is possible to save for this (as plant obstacles grow back every 8 hours), but you would have to have the patience of a saint. With this it's pretty much a case of shelling out the cash to purchase the Gems.
Builders are, of course, important for constructing the wealth of items that you will need to produce units, gather resources, and defend your clan. They are also required for upgrading buildings too. You can see your current number of builders by looking in the center of the resource bar.