It means your domain is locked against transfer, and that is what you want. The status is set by your registrar at your request, and it prevents the domain being moved to another registrar without an extra unlocking step first.
Why it exists
Without the lock, a transfer needs considerably less to go through. The lock protects against someone with access to your registrar account moving the domain away, and against an accidental transfer started during a supplier change or a website rebuild.
When you would turn it off
Only when you are genuinely transferring the domain somewhere. Turn it back on when the transfer completes — that last step is the one people forget, which is why a long-held domain often turns out to be unlocked years later.
Statuses that are not good news
redemptionPeriod and pendingDelete mean the domain has already expired and is in recovery. serverHold means the registry has suspended it, usually over unverified contact details. Those need attention today, unlike clientTransferProhibited, which needs nothing.
This is the short version. The full guide covers it properly: read more.
Every status code, explained: the domain status codes field guide.