Opportunistic:Match Mode: Opportunistic (or -non-strict mode) is the default option and works for most people in most circumstances. Opportunistic matching means that FileBot will do it's utmost to guess the best match for each file based on the information available in online databases (e.g. TheTVDB or TheMovieDB) and assumes that the episode or movie that each file represents actually exists in the online database.
In those rare cases where the filename does not make sense (e.g. trying to match Firefly 2x01 which does not exist according to TheTVDB) the "best match" will still be a bad match since there is no correct match in the first place.
Even though Opportunistic matching works 99.99% of the time, it is not guaranteed to work at all times in all circumstances, as such manually checking matches before hitting Rename is highly recommended. If you find a bad match, just hit the DELETE key to remove the selected match 
from the list.
Strict:Match Mode: Strict is useful when processing large numbers of files that are already fairly well-named. Strict matching means that FileBot will do it's utmost to avoid erroneous matches and ignore files where the correct match cannot be guaranteed. Consequently, in Strict mode there is no user-interaction or any kind of manual selection since files that do not contain enough information (e.g. movie name without year) are ignored right away.
Strict matching guarantees that each match is correct, so checking matches before hitting Rename is not necessary. When processing a large number of files, it's recommended to first use Strict matching to process all files that can be processed easily without user-interaction, and then in a second pass use Opportunistic matching only for the remainder of files that require more attention.