A spot is a short report that a station was heard on a certain frequency at a certain time. Spotting networks collect these reports and publish them, so you can see where the activity is without tuning across the band, and which paths are open from your location. They are one of the most useful tools for HF, DX and portable activations. How well each works depends on its volunteers and their equipment; check each service's own documentation.
Where spots come from
A spot says 'heard here'. It says nothing about stations that nobody was listening for.
Human spots. An operator hears or works a rare or interesting station and posts it. The best-known system is the DX cluster: a network of servers that distribute spots to subscribers, typically through a logging program or a web page. Programs such as Parks on the Air and Summits on the Air have their own spot pages, where activators often post themselves.
Automatic spots. A receiver runs a decoder and uploads everything it decodes. The Reverse Beacon Network does this with "skimmers", chiefly for CW and RTTY (see CW contesting and the Reverse Beacon Network). PSK Reporter collects reports from software that decodes digital modes. WSPR stations report each other's low-power beacon signals; see WSPR.
What they tell you
You want to know
Look at
Is a rare station on the air, and where?
DX cluster
Is my signal getting out, and where is it heard?
automatic reports of your own call
Does this band open to that region now?
a map of automatic reports
Where are the activators today?
the activator spot pages
An automatic report of your own signal is useful feedback: it shows how far a given antenna or band reached, with no need for someone to be listening for you.
Reading them wisely
A spot means "heard there". A signal that skimmers heard strongly may be inaudible at your station.
Absence is not evidence. No spots may just mean no receivers or no operators in that region.
Human spots can be wrong: mistyped call signs, wrong frequency or busted spots that a decoder misread. Confirm by listening before you call.
Receiver density follows population, so activity near big cities is over-represented.
Be considerate when you spot yourself: follow the rules of the service and do not post if you are not actually on the air.
For how to behave once you find a rare station, see DX etiquette and split operation. In a contest, spotting assistance is a separate category; check the contest rules.