It is one thing to swim physically, but it is another thing to swim mentally. Swimming mentally means that you are ready to swim. That you are mentally ready to get in the pool and swim your workout. Swimming mentally is what wins races.
What does swimming mentally do?Edit
While swimming an event, lets say the 50 freestyle, the main mentality is that you want to win. In order to win an event, you have to beat your opponents. Many people give up trying when they notice that their opponent is far ahead and uncatchable. Before standing on the blocks, before going to the swimming meet, before even practice, a swimmer needs to prepare for the meet mentally.