The Downeaster "Alexa" is not actually a thing for 5-year-olds, but I will try my best to explain it to you in a simple way!
Imagine the Downeaster "Alexa" like a really big boat that travels on the water. It is named after a person named "Alexa," just like how a boat might be named after a person. This boat is not your typical boat that you might see people using for fun. It is much bigger and stronger, and it is used to transport things from one place to another.
The special thing about the Downeaster "Alexa" is that it is used to transport goods and cargo, like food, furniture, and toys, across the ocean. It is like a really big truck, but instead of driving on roads, it sails on the water. This boat is designed to carry a lot of stuff, so it has a lot of space where workers can put all the things that need to be moved from one place to another.
Now, you might be wondering, why is it called the Downeaster "Alexa" and what does "Downeaster" mean? "Downeaster" doesn't have anything to do with going down or getting sleepy. In this case, "Downeaster" is actually referring to people from a special place called New England in the United States. They are fondly called "Downeasters." The name "Alexa" is just chosen by the people who made the boat, as it is a popular name.
So, in summary, the Downeaster "Alexa" is a big boat that carries a lot of things from one place to another. It is named after a special group of people called "Downeasters," and it sails on the water just like how a truck drives on the road. It helps to move things across the ocean so that people in different places can have the things they need.