Explorer Skills are upgrades, which can be added to an Explorer to improve their skill at tasks which they perform.
The Skill Tree can be accessed by clicking on an Explorer in the Star Menu, then clicking the 'Skill Tree' button at the top right of the Explorer window, shown below.
The other three icons, with numbers, display how many skills of each level the Explorer already has, in other words, how many books have been used to add skills to the Explorer. The maximum number of skill points is 21 (5 Manuscripts, 10 Tomes and 6 Codexes).
There is a restriction in the skill point distribution: we can give 1 point to the uppermost row, and 5-5 to the others (5 × 4 + 1 = 21). – It's five row in all. The distribution of the 5-5 points is the requirement to reach the next higher row. (And there is a character level requirement too.)
With the rest four button we can search for sectors, adventures, treasures and (since the 25th november 2014 update) expeditions. ( ^__^ )
|1st point||2nd point||3rd point|
| 1st row
(level req. 26)
|Fearless Hiker||Shorten the search time for all adventures by X%.||5||10||15|
|Lucky Detour||Increases the chance of finding Granite on Medium Treasure Searches to X%||8||16||24|
|Pilgrimage||Shorten the search time for Long Treasure Searches by X%.||5||10||15|
|Mountain Boots||Shorten the search time for all Treasure Searches by X%.||3||6||9|
| 2nd row
(level req. 31)
|Deforestation||Find X% more Exotic Wood on Very Long Treasure Searches.||8||16||24|
|Offbeat Roads||Increases the chance of finding Titanium Ore on Long Treasure Searches to X%.||5||10||15|
|Wild Determination||Find X% more Map Fragment on Adventure Searches.||30||60||90|
|Extended Weekend||Shorten the search time for Medium Treasure Searches by X%.||5||10||15|
| 3rd row
(level req. 36)
|Loot Wagon||Find X% more Granite on Medium Treasure Searches.||8||16||24|
|Sturdy Shovel||Find X% more loot on all Treasure Searches.||3||6||9|
|Mistwalker||Raise chance to find Exotic Wood on Very Long Treasure Searches to X%.||5||10||15|
|Travel Expenses||Reduce the cost of all Adventure Searches by X%.||10||20||30|
| 4th row
(level req. 41)
|Hero Ore||Find X% more Titanium Ore on Long Treasure Searches.||8||16||24|
|Trouble-seeker||Gives a X% chance of finding two Adventures at once.||10||20||30|
|Sabbatical||Shorten the search time for Very Long Treasure Searches by X%.||5||10||15|
|Pathfinder||Shorten the search time for all Searches by X%.||5||10||15|
| 5th row
(level req. 45)
|Sophisticated Pillager||Grants a 30% chance to receive an Adventure bonus quest. Completing the Adventure with the bonus quest will give additional rewards.||–|
|Streetwise Negotiator||Gives a 30% chance of a Treasure bonus quest. A bonus quest rewards Granite, Saltpeter or Titanium Ore if found with the Long or Very Long Treasure Search.||–|
|Travelling Erudite||Find Buffs on the "Artefact Search".||–|
|Bean-a-Colada||You might find Magic Beans or Guild Coins on the "Rarity Search".||–|
Rich details for the skill effects you can read in the Explorer skill tree article on The (German) Settlers Online Wiki.
There is a useful calculator in the oSettlers.com, which can help in the point distribution.