When a man's penis or a foreign object is about to be inserted into a female partner's vagina. This is the step before the vaginal occurs. Whether or not this is occurring in an imminent rape (against the woman's volition) or imminent sex (consensual sex) is not important here.
This is different from grinding. There, the female's pussy is rubbing against the penis. Here, the penis is actually being guided into the vagina. The key visual difference is whether or not the penis' foreskin is positioned just outside the vaginal cavity.
Also, try not to confuse this with after vaginal, which may look similar to this in small thumbnails.
Likewise, it can count as sex if the female partner is already in the middle of anal sex with one male partner, while a second male is about to put his penis in her vagina. See imminent double penetration for further details.