This is a short hand way of describing contact configuration.
A “Form A” would mean the contacts are normally open when the coil of the relay is not energized or the there is no magnetic field nearby in a reed switch.
A “Form B” would mean the contacts are normally Closed when the coil of the relay is not energized or the there is no magnetic field nearby in a reed switch.
A “Form C” would have 3 leads and would have 1 normally open and 1 normally closed circuit. This is also called a “changeover” device because the common contact changes from the normally closed position to the normally open position when the coil is energized in a relay or a magnetic field is nearby in a reed switch.
Here is an example of the contacts in there non-energized state.
Below would be the same contacts in their energized state.
You will also find many relays that have multiple contacts and some will have a variety of configurations. You can select the option you need under the contact form section(shown below).