A kli sheini is the vessel that holds food which was poured from a kli rishon. For example, if one boiled water in a pot and poured the water into a cup, the pot is a kli rishon and the cup is a kli sheini. By pouring the liquid from a kli rishon into a kli sheini the liquid gets cooled down a bit and loses its power to cook. The Shulchan Aruch and Rama (יו”ד קה:ב) rule that after the fact, one only needs to rinse a food that became forbidden in a kli sheini. Meaning, if a piece of meat falls into a kli sheini of hot milk, the meat needs only to be rinsed before eating it. The milk is permitted, even though optimally you need to rinse both the meat and milk, since it is impossible to rinse off milk it is permitted after the fact. The Shach (שם ס”ק ה) says that optimally one should be stringent and take off a kelipa from the meat, but when that is not possible or if it will incur a loss to remove a kelipa, one can be lenient.
In case of a doubt if something fell into a kli sheini one should consult with a rabbi, as there is room to be lenient.