What does "external reviewer" mean for a computer science conference?
Conferences are organized differently in different subfield of computer science; my answer describes the situation in theory.

At theoretical computer science conferences, the program committees are relatively small, in part because submissions from program committee members are strictly forbidden.  Each submitted paper is assigned to 2-4 PC members to review; for a typical conference this means each PC member is assigned 25-50 papers to review.  Reviewing that many papers is a huge burden, and despite a preliminary bidding process, every PC member is assigned at least a few papers outside their areas of expertise.  So PC members regularly solicit reviews from people outside the committee whose expertise better matches a given paper.  Those people are external reviewers.  

Normally each PC member tries to gets a different external reviewer for each paper (except the papers they review on their own), but a single external reviewer might get papers from multiple PC members.

(In other communities, what I call "PC members" are called "area chairs", and what I'm calling "external reviewers" are called "PC members".)


  Does this mean the person officially reviews the paper for the conference, or he/she gets a request to review from someone who officially reviews the paper?


Both.

A list of all external reviewers is typically included in the conference proceedings just after the table of contents (for example) as official acknowledgement of their service.

On the other hand, PC members are expected to use the external reviews they solicit as guidelines, not as substitutes for their own judgement, especially when comparing submissions that were reviewed by different people.