Hey, it's great to know you want to find out more about it. Digital marketing is quite a broad spectrum, obviously you have marketing channels such as SEO, PPC but then you can branch out onto things such as content marketing to help with your strategy and create content such as blogs and web copy that will engage your audience. You've also got web development and graphic design, so you can program and create websites and graphics that will enhance your product or service, then there is social media and how you can outreach to your audience on social media channels and get involved in conversations. Social is also a great way to promote things as well. These can be accessible via mobile phones, tablets etc. Digital marketing enables brands to advertise through different forms, whether this is a device such as a mobile or one of the channels I have mentioned above.
As digital marketing is a huge and diverse topic, I'd recommend reading up on it to help you understand how granular it is. I can't send links at the minute, but a website called smart insights provides some great information about what digital marketing is. I've just covered the main parts, but I hope I have helped