Typically this refers to a Network-Based attack, where remote nodes all flood a server with requests, in the attempt to overload the server or network with requests, taking it down for everyone else on the network. This is typically done using botnets, of compromised nodes, in a coordinated way
Mitigating Network DoS attacks
Static fierwalls are used to prevent/block certain ports and IP ranges, known to be bad actors. Signature-based Network Firewalls (snort) Anomaly-detection based Firewalls (machine learning)