Truncation is when a voter votes honestly, but doesn't show some of their lower preferences. In the extreme case, it becomes bullet voting, where the voter only indicates their first preference. It is often done as part of a voting strategy, although it can also be done to express indifference.
One risk of truncation in ranked-choice voting is "ballot exhaustion", due to all of the expressed preferences being eliminated in early rounds of said election. The voter may have had the opportunity to express their preference between some of the less preferred candidates in the election, but chose not to express a preference by leaving lower rankings blank. Note that in some jurisdictions that use ranked-choice voting, voters may be limited to three or four rankings for their favorite candidates, and may not have room on their ballot to rank the candidates who advance to the later rounds of counting.
A variant of the Participation criterion or Independence of Irrelevant Ballots offers one way of justifying score voting being no worse than FPTP: voters can never be hurt by casting an FPTP-style vote (bullet voting) relative to not voting. Many other voting methods, such as IRV and STAR (and possibly Condorcet) fail even this weaker version of Participation. Example for IRV:
30+2: A>B 31: B 49: C
If 2 of the 32: A>B voters show up and bullet vote A, then B is eliminated and then C wins. But if they don't vote, then A is eliminated and then B wins.