A solar battery is a power storing device that can be charged by the energy your solar panels generate and saved for later use during times your solar system has not produced enough energy to power your needs. This is common during low sunlight times such as early mornings and evenings.
Depending on your individual circumstances and power usage, more often than not you will find your solar system will produce more power than you are using. Without a battery storage device, this excess power gets fed or sold back to the grid and your energy provider will give you credits or sell it back to you when you need it. This sounds all well and good, however, your electricity provider will almost always sell the power back to you at a much higher rate than you sold it for.
With a battery storage system in place, rather than selling your excess power back to the grid, you store it on your battery for use whenever you need it during periods of low light conditions, which reduces your dependency on the grid making the battery storage option a more cost effective solution in the long term. The only time you will need to draw power from the grid is when your battery is depleted.