ss-lock means subshared lock. It allows unprotected reading. If you run for example a 2-node OPS/RAC, node 1 might hold the lock on a block in ss-mode and node 2 may be able to read and even modify the block.
s-lock means shared lock. The difference between ss-lock is that no instance can write/modify the block. Just for reading.
If you are curious, the rest of the locks are null-locks, sx-lock (shared exclusive), ssx-locks (subshared exclusive) and x-locks (exclusive).
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