The Lord said to Moses and Aaron in the land of Egypt: This month shall mark for you the beginning of months; it shall be the first month of the year for you. Tell the whole congregation of Israel that on the tenth of this month they are to take a lamb for each family, a lamb for each household. If a household is too small for a whole lamb, it shall join its closest neighbor in obtaining one; the lamb shall be divided in proportion to the number of people who eat of it. Your lamb shall be without blemish, a year-old male; you may take it from the sheep or from the goats. You shall keep it until the fourteenth day of this month; then the whole assembled congregation of Israel shall slaughter it at twilight. They shall take some of the blood and put it on the two doorposts and the lintel of the houses in which they eat it. They shall eat the lamb that same night; they shall eat it roasted over the fire with unleavened bread and bitter herbs. Do not eat any of it raw or boiled in water, but roasted over the fire, with its head, legs, and inner organs. You shall let none of it remain until the morning; anything that remains until the morning you shall burn. This is how you shall eat it: your loins girded, your sandals on your feet, and your staff in your hand; and you shall eat it hurriedly. It is the passover of the Lord. For I will pass through the land of Egypt that night, and I will strike down every firstborn in the land of Egypt, both human beings and animals; on all the gods of Egypt I will execute judgments: I am the Lord. The blood shall be a sign for you on the houses where you live: when I see the blood, I will pass over you, and no plague shall destroy you when I strike the land of Egypt. (Exodus 12:1-13, NRSV)
Eat the lamb you roasted over the fire with bitter herbs with your loins girded, your feet in sandals, staff in your hand, ready to leave.
You see a man would not gird his loins until he was ready to run. So God is telling the Israelites that this is going to be a quick meal that they will hurry to eat.
God gave the first fast food to the Israelites! Roasted lamb with bitter herbs and unleavened bread.
And you need to use the blood from the sacrifice of the lamb so that God will know what houses to pass over. So those in that house will be safe.
When I see the blood, I will know you are ready and I will passover and not plaque that household!