Kerning means adjusting the spacing between pairs of letters. When font sizes are large, you'll often notice that the space between some letters seems wider or narrower than the spaces between other letters .
To even up the spacing, you can kern by placing your cursor between two letters, holding down the Option (Alt) key, then pressing the Left Arrow key to move the letters closer together or the Right Arrow key to move letters further apart. Since you can also use the Left and Right Arrow keys (without the Option or Alt key held down) to move the text cursor, you can quickly make many kerning adjustments by moving the cursor with the Arrow keys and adding in the Option (Alt) key when you want to kern.