We use mostly use batteries in our life and we connect them to two connections, connecting batteries in series is a connection which we use mostly when we have two 12 volts batteries and we required the 24 volts DC supply.
Also, Read Below :
How do wire Batteries in Parallel?
How To Wire 2 Batteries In Series?
Batteries in Series Connection
In series batteries volts will be plus, e.g if you have two 12 V batteries and if you connect both series then your total volts will be 24 V, and Ah (ampere hour) will be the same.
So if you required 24 V DC then you can easily get it from two 12V batteries by connecting both in series.