In order to understand the stratum protocol, it might be helpful to understand what came before it, which was the RPC protocol. RPC stands for Remote Procedure Call. In cryptocurrency circles it is often referred to as "getwork". It means the miner continuously queries the mining pool to see if any of the mining parameters have changed. Some mining operators configure their software to poll the mining pool multiple times per second. Obviously, with many miners, this is very inefficient and places a heavy load on the mining pool.
The Stratum protocol, on the other hand, avoids continuously polling the pool by establishing a persistent TCP connection with the mining pool. (It's like phoning someone and keeping the line open.) Once the link has been opened the mining pool sends the current mining parameters to the miner. The TCP link is kept open, and whenever the mining parameters change, the mining pool simply sends them down to the miner.
The Stratum Proxy is like an interpreter that speaks two different languages. When it starts it establishs a link to the mining pool at http://mvis.ca:8090 using the stratum protocol to obtain the relevant mining parameters. At the same time, it also configures itself as an RPC Server and begins listening for RPC requests from your mining software. It provides the necessary mining parameters to your miner when asked, and receives share submissions from your miner and passes them up to the pool. Your mining software will think it is connected directly to the mining pool.
The basic idea is that you are awarded tokens on a regular basis, every 4 hours, regardless of whether the pool mines a block or not. This is in contrast to the Proportional payout scheme used by a lot of pools, where you only get tokens when the pool mines a block.
If the mining pool hits a run of bad luck and doesn't mine a block in a while, a portion of your rewards may be held back depending on the current pool balance, but it will be kept on your account and paid back later when the pool is having better luck.
A holdback is when tokens that you are owed are transferred into a special holding account instead of being paid out to you. This happens when the pool balance runs low due to a spell of bad luck.
Remember that as a miner, your token rewards are calculated assuming that the mining pool will be able to mine blocks at a perfectly consistent rate, but of course it doesn't work that way in real life. The actual profit of the mining pool is subject to variance, or to put it more simply, luck. If the mining pool has a run of bad luck and doesn't mine as many blocks as expected, the pool token balance can run low. When this happens, a portion of your mining rewards will be held back and put in a special holding account. Later, when the pool has better luck, the tokens will be paid back to you.