In Signal, players take on the role of Experts seeking to understand the Alien player, who in turn is seeking to be understood. The Experts will create and transmit "signals" to the Alien, who has certain rules for how to respond that must be deduced by the Experts. Once the Experts understand these rules, they can ensure that they come to agreement with the Alien, succeeding co-operatively.

On a turn, the Experts arrange black and white pieces on a mat to represent a graphical signal, discussing and debating how and what to send. Once finished, the Alien then replies by using pre-set rules only they can see to add, remove, and adjust the signal, both to be helpful and to try to reach an agreement, which is represented by a pre-set signal that they are trying to achieve with their limited response capabilities. As the Experts analyze and understand the Alien, they can use limited guesses and a few hint-like abilities to discover which signal they can send that will result in agreement. The Alien even has a little leeway in how they respond; after all, they want to be understood.

After succeeding at the first round, Experts then reset and continue with the same Alien, except new rules are in play on top of the previous ones, showing a deeper and deeper familiarity with the Alien player. After three rounds of successful agreements, all players win co-operatively and receive a score based on leftover guesses and unused abilities.

—description from the publisher

The publisher has issued errata to correct errors on three of the Alien cards.