The support will build basic strength to keep yourself up during L-sits and to perform dips. The rings support will also build some chest strength (the chest acts as a stabilizer) and condition the biceps tendon for more stressful endeavours like the back lever.
In it’s most basic form, a support is holding yourself up. This can be with rings, a single bar, or parallel bars.
Rings Turned Out Support Hold (RTO Support)
Find two chairs of equal height and use those as parallel bars. If you’re afraid of them falling over, put some weight on the seats. Another option would be to use a counter top that makes a 90 degree angle, or two tables. Whatever works.
If you don’t have rings, the progression ends at Parallel Bar Support.
Is it okay if my forearms touch the rings?
That depends. If you’re using your forearm to lean on the rings, then no, that’s not okay (you see this in a lot of ring training videos, actually). Otherwise it doesn’t matter much.