Why was the Battle of Trafalgar such an important military loss for Napoleon?
The Battle of Trafalgar essentially crushed Napoleon's hopes of establishing naval superiority over Great Britain. The battle resulted in the destruction of twenty-two Spanish and French ships compared to no such loses by the British. Although the French continued their shipbuilding program and increased their navy by utilizing the fleets of the nations that they conquered, they never again directly challenged the British at sea at a large level. After the battle, the British navy instituted a decade-long blockade of French ports which greatly limited their ability to rebuild their fleet.
Even before the Battle of Trafalgar, Napoleon had abandoned his plans to invade Great Britain. After the battle, any discussion of reviving these plans was ended. The defeat so demoralized the French that they refocused their attention entirely on land campaigns, which, for the most part, they had great successes with. However, with Great Britain ruling the seas, they could only expand their empire so far and were hampered by an inability to travel and trade freely at sea.
The Battle of Trafalgar was an important part of the Napoleonic Wars. It was an important military loss for Napoleon because it prevented him from invading England and because it guaranteed that the British would retain naval supremacy for the remainder of the war (a major reason they were able to win the war).
When the Battle of Trafalgar happened in 1805, France was the strongest military power on the continent of Europe. By contrast, Britain's major strength was naval. If the French could break the naval strength of the British, the power of their army would likely have been used to invade England.
At the Battle of Trafalgar, the British destroyed much of the combined French and Spanish fleets. By doing so, they kept naval superiority and made a French invasion of England impossible.