Here are the three possible results of a battle:
Win: You captured the pirate for this expedition (island). The pirates will get him back for the next expedition.
Lose: The pirate captured your nav for this expedition (island). You'll get him back for the next expedition.
Tie: Neither one was captured, but both got more tired (lost energy). After one or two ties you might try attacking the pirate with a fresh nav now that you've made him weaker.
The Attack/Defend dialog tells you:
1) In the larger print: The chance your nav will win the fights that do not end in a tie. In the above example, there is a 49% chance that the nav will win the 40% (100% - 60%) of the fights that do not end in a tie.
2) In the smaller print: What chance the fight will end in a tie. In the above example there is a 60% chance that the fight will end in a tie.
Display Expert Options
Select the Display Expert Options option if you want to have more options in attacking. If you choose this option make sure you understand the choices or you may actually hurt yourself. Here's what the attacking and defending combat dialogs with expert options looks like.
How Combat Strength is determined