When we analyze voting systems, there are some concepts that we must introduce, and some assumptions that we must make. The assumptions are not always true, but sooner or later, they often become true, and if not, then we are specifically aware of them not being true.
When we say sincere preferences, we are referring to what the voters actually desire, as opposed to the preferences they record on the ballot. In most voting systems, there is a discrepancy between what voters want and what they end up marking on the ballot.
When analyzing voting systems, we assume that voters are thinking short-term. That is, we assume that voters are trying to get the best possible result this election, without regard to affecting future elections. This is sometimes not true; for example, Ralph Nader frequently appealed to voters to vote for him "in order to build a party", i.e., to affect future elections. However, this is the exception that proves the rule: in order to get people to think long-term, he had to make an explicit request.
We assume voters are rational. That is, we assume that they know and will use the best strategies they have of achieving results that conform to their sincere preferences. (This says nothing about the rationality of those sincere preferences.) In many cases, voters do not initially know what their best strategy is, but if they vote in the same system long enough, and if other people around them get a chance to speak to them, especially candidates whose campaigns will be affected, they will eventually understand what their best strategy is. For example, in the 2000 presidential election and the 2002 Massachusetts gubernatorial election, the Democratic Party expended significant resources informing voters about their best strategy, using signs like "Stein = Romney" and more sophisticated literature.
When analyzing the results of an election, we must very carefully consider the quality the information voters have about other voters' preferences. If there are no polls to base support on, voters will estimate the support each candidate has on other information, e.g. past performance of the party the candidate is affiliated with, candidate credibility, et cetera. If there are public opinion polls, then voters may change their voting strategy based on those polls.