English: We learned in a mishnah, R’ Meir says: Anyone who forgets even one item of his learning — Scripture regards him as if he is liable for his life, as it is said “and watch your soul very much, lest you forget the things that your eyes have seen.”
On the daf / in context: The derasha applies only when the forgetting results from negligence (failure to review); if the material is simply too difficult, the verse itself limits the liability (“and lest they depart from your heart” — only when one actively removes them).
Type of Learning
Diyuk of “pen tishkach… asher ra’u einecha” — forgetting through neglect is treated as a capital-level failure.
What Is Learned
One who, through neglect, forgets even a single item of Torah is regarded by Scripture as having endangered his life; constant review is therefore obligatory.