Definition of borderline
In poker, a winning player will make what he calls EV+ (Expected Value Positive) plays, which are plays that have a positive long-term payoff expectancy. Pushing all in preflop with a pair of aces is an example. In the long run, you know that this play will make you money (EV+). Bad players often tend to make more EV- (negative payoff) games than EV+. But some games will have a neutral or marginally positive payoff expectancy, these are called borderline games.
Example of a borderline game
With poker solvers gaining popularity in recent years, we now know which games have a marginal or borderline payoff expectation. In some situations, you may find that a bluff on the river is borderline. The pot is big, you have a very weak hand that won't win on the showdown, and you're evaluating the option of bluffing. By calculating what you will lose when your bluff is called and what you will win when your bluff works, you calculate that your bluff will win you $2 for every 100 times this situation occurs; this is a classic example of a borderline situation.