Like terms is any group of variables or numbers which are similar. For example, 3 and 5 are like terms. We can add these two together no problem!
However...
What about 3x and 5? Well... We can't just add them. You see, the 3 has a variable attached to it. This makes it incompatible with adding it to the 5.
But...
What about 3x and 5x? Yes! We can add these because they're like terms! The answer would be 8x.
But then again, 3x and 5y are not like terms. We can't add these two together. We can add 5y and 3y together because they're like terms. But 3x and 5y aren't.
We can't add 3x and y together, because again - they're not like terms.
We could, however, say, add 3y and y together, because well... A variable on its own is the same as 1y, or 1x, or 1z, or any darn variable! There's an imaginary one there, that we just don't list. =)
