You should be looking for litter of all kinds, such as plastic wrappers, bottles, polystyrene, cotton bud sticks, even rope. Keep an eye out for nurdles too- small plastic pellets.
If you encounter something sharp such as glass or nails, call your organiser over and it will be handled separately.
Organic material such as seaweed and driftwood (not wood that is painted or has nails sticking out) can remain on the beach as they are important to various organisms living there and will also biodegrade safely.
After the cleanup, recyclables will be separated out.
Other litter will likely be bagged up and disposed of appropriately via the organisation or council.
Sometimes litter bags will be weighed by the organiser.
Any information obtained by the organiser such as sheets you fill out might contribute to reports.