what is mean by gallon ?

Dear student, 
​​​​​​a United States unit of liquid capacity equal to four quarts or 231 cubic inches or 3.785 liters.

a British unit of liquid and dry capacity equal to four quarts or 277.42 cubic inches or 4.544 liters. — called also imperial gallon.

The gallon is a unit of measurement for volume and fluid capacity in both the US customary units and the British imperial systems of measurement. 

Regards