In a nutshell, battery voltage is the overall power the cells of the battery generate when you power on the tool. Is a higher voltage better for cordless tools? The tool and task will determine your voltage needs. Lower voltage tools and batteries are suited for small and simple tasks like cutting or drilling into soft woods or the occasional around the house maintenance requirement. Low voltage batteries are smaller, cheaper, and charge quickly.
Whereas, larger tools and tougher jobs typically require a higher voltage (20V or more), this translates to higher torque, which makes it easier to generate greater force without straining the battery. High voltage tools and batteries come with more torque and power, at the trade off of higher costs and can weigh more.