It is like we say I am here and I am there. here=ici and there means là

Let's consider two examples. You work alone at home. Your spouse comes back from outside, and casually shouts : - Chéri, tu es là ? - Oui, je suis là. - Mais où es-tu ? - Je suis ici. Your spouse asks whether you're at home or not. Your first reply means that you're home indeed, while the second one refers to your specific location — let's say, the kitchen. A friend happens to be in your neighbourhood and wants to drop by. He phones you and asks: - Tu es là aujourd'hui ? - Non, je ne suis pas là. Your friend asks if you're at home, and you answer negatively: maybe you're at work, or gone shopping, or taking a walk. « Être là » needn't refer to home. It could be any implicit location. In the first dialogue, your wife obviously wants to know if you're at home ; so does your friend. If you're often on the road for professional reasons, a colleague may ask: - Tu es là aujourd'hui ? Meaning : are you at the office ? The implicit location is the office.