Other people already explained the base case, but also: for voters who are out of the town were they originally enlisted, it just a matter of going to any poll station and filling a justify form. It is usually quicker than voting but, as the voter can not do it at the enlisted town, it is not an alternative.
Voting is also highly distributed, on most towns no voter has to travel more than 1 or 2 km to get to their assigned polling station (if they enlisted to the station nearest to their homes, of course). Every voting zone is split in a number of "voting sections", every one with a single voting machine, and the number of section/machines per zone is mostly proportional to the number of voters, so queues are not that common; unless people group in some particularly unfortunate times ... local jokes say to NEVER vote at the opening hours, or you will be stuck on a line of senior citizens (who also have the right of jumping the queue, defined by law).
Also, everything is defined with months of advance, so there are no surprises on where to vote.
There is a lot of safeguards to prevent fraud (we had a lot of time to learn this by the hard way) but the bad part is these also prevent some nice things (quickly moving your enlistment to another town/zone/section, mail voting, etc... even transit voting is very limited because you will need to know where you will be long before the poll).
If you miss a few elections, you get put into a blacklist and can't get a passport, work for the government and a few other things.
However, the fine for missing an election is ridiculous. It's something like 1 dollar. If you don't want to vote, you can stay at home and then pay this fine, but there is a caveat: 100% of this fine's value goes to Fundo Eleitoral, which is a fund managed by the parties and meant to be used for their publicity. In other words: you put money in an instrument that's probably corrupt.
If you don't pay the fine you can't apply for public jobs, if already a government employee you can't receive a salary, can't issue a passport, and other restrictions. Nowadays you can justify the absence using a cellphone.
You just have to file a note explaining that you could not vote -- we call it "justifying the (non-) vote" / "justificar o voto". As of the current election, the it can be done on a mobile app.
What happens if you don't vote?