A crush (also man crush or woman crush) was a feeling of love or romance towards another individual. This was quite a powerful force. As the Twelfth Doctor expounded, "Never underestimate a crush." (TV: The Pilot)
Bill Potts and Heather developed a mutual crush on each other after they met at St Luke's University. At one point, when Heather asked her to come around so they could look together at a strange puddle, Bill made her promise not to leave without her.
After Heather was absorbed by the puddle of sentient oil, her last conscious thought drove the creature to follow Bill across the universe, into both the future and the past, to withstand the blast of a Dalek gun, all to invite Bill along with her. Once Bill realised that Heather was still bound by her promise, she released Heather from that promise, and the two had to let each other go. They both admitted that they had really liked each other, and before saying goodbye, shared a few moments of cosmic wonder by simply holding hands. (TV: The Pilot)