What is 'adj' in train_dataset[0] in PyTorch codes?

I am learning PyTorch. In some codes such as this one, I have seen there is a snippet that I do not understand how it works:

from torch_geometric.data import DataLoader, DenseDataLoader as DenseLoader

if 'adj' in train_dataset[0]:
                train_loader = DenseLoader(train_dataset, self.p.batch_size, shuffle=True)
                val_loader  = DenseLoader(val_dataset,   self.p.batch_size, shuffle=False)
                test_loader = DenseLoader(test_dataset,  self.p.batch_size, shuffle=False)
            else:
                train_loader = DataLoader(train_dataset, self.p.batch_size, shuffle=True)
                val_loader  = DataLoader(val_dataset, self.p.batch_size, shuffle=False)
                test_loader = DataLoader(test_dataset, self.p.batch_size, shuffle=False)

First of all, what is if 'adj' in train_dataset[0] and secondly what is the difference between DenseLoader and DataLoader?