Rear Admiral is a naval commissioned officer rank [...] below that of a Vice Admiral. It is generally regarded as the lowest of the "Admiral" ranks[.]
It originated from the days of naval sailing squadrons and can trace its origins to the Royal Navy. Each naval squadron would be assigned an admiral as its head, who would command from the centre vessel and direct the activities of the squadron. The admiral would in turn be assisted by a vice admiral, who commanded the lead ships which would bear the brunt of a naval battle. In the rear of the naval squadron, a third admiral would command the remaining ships and, as this section of the squadron was considered to be in the least danger, the admiral in command of the rear would typically be the most junior of the squadron admirals. This has survived into the modern age, with the rank of rear admiral the most-junior of the admiralty ranks of many navies.
Edit: just wanted to add this important bit from another article:
The Royal Navy rank of Rear Admiral should be distinguished from the office of Rear-Admiral of the United Kingdom, which is an Admiralty position usually held by a senior (and possibly retired) "full" Admiral.