SMALL BUT MIGHTY

I'm small and often missed,

I help you when you list;

When you see me, you should pause,

To honor grammar's laws.

You may dismiss me as having little weight,

But my absence misunderstanding can create;

On the one hand you may say, "Let's eat bread."

But I hope you never say, "Let's eat Ted."

Answer:

A comma (should be, "Let's eat, Ted.")