Yomi means "reading" in Japanese. In gaming, "yomi" refers to "reading your opponent." The word's use in gaming seems to have been introduced by David Sirlin, who designed the card game "Yomi" and uses the term to describe the concept of every action having an action that counters it. The play/counter-play in Sirlin's card game was inspired by the video arcade game "Street Fighter" and its attack/parry/grab mechanic.
In SpyParty, almost everything has some level of yomi. No single strategy can guarantee victory because every choice has a corresponding countermeasure. The better that a strategy is, the more the opponent should expect it, watch for it, and play against it. Players may try the "good" strategies—despite being just what their opponent would expect—or they may try supposedly "bad" strategies to catch their opponents off guard. A good Spy and Sniper should always be trying to outguess each another.
A clear example of yomi in SpyParty lies in the Spy's character choice. Suppose that I am playing as Spy and that I believe that I can almost always win if I play as Smallman. Then I may think that I should always choose to play as Smallman for the best chance to win. But if my Sniper opponent also believes that Smallman is very strong, they will expect me to be Smallman and watch him very carefully for tells. This will make it more difficult to complete missions unnoticed, which decreases Smallman's chances of winning. The Sniper's reaction to Smallman's strength pushes back against that strength. The power of good Spy characters is countered by the extra scrutiny that they receive.
To summarize, being good as a Spy makes a character the obvious choice, and obvious Spies get shot.
Yomi may also refer specifically to adaptations that players make due to knowing one another's preferences. Suppose a Spy knows that their Sniper opponent often shoots Civilians. The Spy may play slowly in order to give the Sniper more time to make a wrong shot. In turn, the Sniper—knowing their own reputation—may guess that the Spy will use this strategy and be reluctant to shoot in hopes that the Spy will run out of time.