Definition of borderline
At poker, a winning player will make what he calls EV + (Expected Value positive) games, i.e. games that will have a positive expectation of winning in the long term. Pushing all in preflop with a pair of aces is an example. In the long run, you know that this game will make you money (EV+). On the contrary, bad players often tend to make more EV - games (with a negative win expectation) than EV +. But some games will have a neutral or marginally positive win expectation, which is called borderlines games.
Example of a game borderline
With the poker-solvers that have gained popularity in recent years, we now know which game has a marginal or borderline payout expectation. In some situations, you may be able to tell yourself that a river bluff is borderline. The pot is big, you have a very weak hand that won't win at abattage, and you're evaluating the bluff option. 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 euros every 100 times this situation occurs; this is a classic example of a borderline situation.