What is bein hashemashot?

Hello and blessings

There is a time about which halacha is uncertain whether it is day or night, as follows

Until sunset it is certainly day; from the time it is dark and three stars are visible in the sky, it is certainly night. From the time the body of the sun is hidden from our sight until the stars are visible, there is a certain period when it is not known whether it is day or night, and this is bein hashemashot.

With blessings,

Binyamin Shmueli