The different type of 'Si'

Once in a while I'll see a sign that says:

Se, habla español.

I think to myself, this must be wrong. Why does it use "se" and not "si"?

For what I understand, this would be correct: Si, hablamos espanol.

Yet, I'm seeing something interesting in my book. I see that it says sí (with an accent) signifies a different word.

What is the correct way to say "yes" in Spanish?

Si hablas español, podrás entenderme.

Si = conditional conjunction (English if)

Ghost Writer

If a business wanted to tell it's customers, "Yes, we speak Spanish," would it post on its door, "Sí, hablamos español"?