This term comes from a Chinese proverb. It means that to communicate heart (or mind) with each other by means of their heart (or mind). In other words, to communicate with each other’s mind without speaking any words.
The first two characters, 以心, are translated as, “by means of heart (mind).” The last two characters, 伝心, are translated as “to communicate the heart (or mind).”
It suggests learning by doing with masters of doing. In the martial arts, the communication is through practice. In the hard dōjō, this means a lot of getting thumped when you do something bad (see shuhari for a bit on the good and bad vs right and wrong). Often we also speak of “direct transmission” which is something that happens when you have been chosen as a deshi and have been practicing continuously for many many years with a master.