Single English words cannot be trademarked. However, if you string two of them together, and demonstrate that you are actively using the phrase in commerce, you can get the phrase trademarked for use in a particular domain, e.g. computer software.
Three words is even better.
Note that a tractor manufacturer could still trademark "Active Custom Fields" for agricultural equipment, because it would not be confusingly similar to the "Active Custom Fields" software.
Also trademarks have to be renewed every 5-10 years and you must show that you are actively using it.