The SRD refers to the System Resource Document, which contains the features, classes, races, spells, monsters, etc.that Wizards fo the Coast has released for free usage. Anything else from the core books is licensed material, meaning that players must pay in some way in order to have access to it. You could purchase the core books, or buy them through various online services (such as Roll20). Unless you purchase the material, you are limited to the free material provided in the SRD, which is whaat you see when you come to Roll20. To date, of the three core books, Roll20 has only managed to procure a license for the Monster Manual. There are other books available, such as Volo's guide or many adventures, but that is the limitation in core rules. This means you have access to the one sample background in the SRD, the one sample feat, one each of each sub-class archetype, a (generous) selection of spells, and so forth. EDIT: sniped. :)