This is one of many random events that can occur when colonizing a planet. These are fully discussed in the Wiki Random Event article. There are actually two different random events that give a Starship Bonus. The one you mention is "Pirate Lair" and there's another called "High-Tech Hijack".

Anyway the appropriate bonus (i.e. the 0 to 60% Starship bonus) increases the weapons and defense of all ships produced on the planet.

There are a number of previous threads where this is fully discussed all of which were very easy to find using a "starship bonus site:https://forums.galciv2.com/" search string at google.

Planetary Effects from Good v Evil choices?

What the heck is "Starship Bonus?"

Colonization Random Events

First time I've been referred to as Mr Mumble.

As far as the comment about easily searchable using site search at Google (site:https://forums.galciv2.com/), I wasn't implying that you should have thought to do this yourself. Personally I never knew of this function until Kryo pointed it out to me. It's more a matter of teaching someone to fish as opposed to giving them a fish. Also the Wiki is a very good reference for a lot of questions. You can most easily access it by hitting the Databanks button in the top left of every page and clicking on the Wiki Site link.

Regrettably it's still difficult to find a specific user's profile using google site search. Sometimes you can find one by searching for the exact string match of "forum post by username" along with the site search string (site:https://forums.galciv2.com/), but very often if a user does not have many posts you'll get no matches. Plus there's no way at all to find replies by a specific user with this method.

It used to be that the in-site search was very useful, however since the forum upgrades it's been virtually useless. Oh well.