What is a tesserae and how does it put certain people at a disadvantage in The Hunger Games?

The answer to this question can be found in Chapter One,
when Katniss tells us about the Hunger Games, how the contenders are selected, and how a
special system allows poorer people to be more likely to be selected than richer people
in exchange for more food for their families. Note how Katniss explains the system of
tessarae:

Say
you are poor and starving as we were. You can opt to add your name more times in
exchange for tesserae. Each tessera is worth a meager year's supply of grain and oil for
one person. You may do this for each of your family members as well. So, at the age of
twelve, I had my name entered four times. Once, because I had to, and three times for
tesserae for grain and oil for myself, Prim, and my
mother.

Thus the system of
tessarae gives more food to the poor in exchange for having a greater chance of being
selected. Katniss goes on to explain how the entries are cumulative, so that each year
you add another four onto the number of entries bearing the name of Katniss. Thus the
system is unfair because statistically, the poor are much more likely to be selected
than the rich who do not need to partake of this system.