Introduction: the present progressive
When you say "I am eating" or "She is working" in English, you are using the present progressive. Spanish has the same tense: the verb estar followed by a gerund (gerundio), the Spanish "-ing" form.
The formula is estar (conjugated) + gerund. You already know the simple present (como, trabajo) for what you do in general. The present progressive (estoy comiendo, estoy trabajando) is for what is happening right now, or around now.
Examples
Unlike English, Spanish does not use the present progressive for future plans. "I'm travelling tomorrow" is "Viajo mañana" or "Voy a viajar mañana", never "Estoy viajando mañana".