Counters are only mentioned briefly in the developer documentation.
Furthermore each operation has a counter (explained in more detail later) that prevents replays so it is usually safe to re-emit an operation that seems lost.
What is a counter in this context?
What is replay and why do we want to prevent it?
I tried to perform operations in parallel with a single public/private key pair in tezos-client and it complained about the counter. If I manually increase the counter in the second operation will it work properly?