[wiml]

It's probably just logging AIS data ( https://en.wikipedia.org/wiki/Automatic_Identification_Syste... ). Large ships are required to broadcast (like, on the radio) their position, heading, and so on. Anyone with a radio receiver can collect this data for ships within radio range. Anyone with a large number of internet-connected radio receivers can collect this data over a large area.

[juskrey]

Amateur AIS receiver networks are only covering near-shore areas, for obvious reasons.

Covering deep sea/ocean areas (often the most interesting part) requires some paid subscriptions.

[Moru]

They often give out free receievers to spread their network