We can write each letter of the English alphabet as a "small letter" (abc) or as a "large letter" (ABC). Large letters are also called "capital letters" or "capitals".
Below you see the whole alphabet with small letters on the left and capitals on the right:
In informal English, we sometimes call capitals just "caps".
Small letters are sometimes called "lower case" and large letters "upper case". This is because in the old days of printing, before computers, the metal blocks for setting type were kept in two different boxes or "cases": small letters in the bottom or lower case, large letters in the top or upper case.