You're right, it's the equivalent of a right handed turn in the UK.
Consider an intersection between two perpendicular streets that is controlled by traffic signals.
A protected turn is one in which the turn through the intersection and across the oncoming lane of traffic happens when the oncoming traffic (that is going straight) is stopped.
An unprotected turn is one in which the turn through the intersection and across the oncoming lane of traffic happens when the oncoming traffic does not stop.
In the unprotected turn's case, traffic going straight has the right of way, so cars wishing to turn must wait for a break in oncoming traffic, or for the signal to change. In either case, one car goes straight into the intersection and then stops and waits with its blinker on for a safe opportunity to finish its turn. If it is still there when the light changes, it completes the turn as oncoming traffic comes to a halt and before the sideways traffic begins to move.
Unprotected turns are usually done when both streets are not very busy or when one street is much busier than the other street. In that case the less busy street will have unprotected turns, because it means that there is one less signal during which the busier street is at a complete standstill (ie one signal for both traffic turning onto the busy road and traffic going straight across the busy road instead of one signal for traffic turning onto the busy road and another signal for traffic going straight across the busy road).
Unprotected turns can also work in on very busy streets (think traffic jams), because the streets are so filled that only one car would have space to complete a turn without remaining partially inside the intersection anyway. This allows the lights to cycle faster, since there are fewer signals per cycle.
The Los Angeles area is (semi-)famous for its many intersections with unprotected left-hand turns.