Why are the proletariat a "genuinely revolutionary class"?
The proletariat are the roots of revolutions throughout modern history and even before that because they are the most routinely exploited and abused peoples of society: slaves, factory workers, field workers, miners - these are professions we think of as dangerous, back breaking and underpaid.
These are the classes of people with the least to lose in a revolt and the most to gain. It is also almost always the largest class in terms of numbers, and it is through numbers, Marx argued, that the proletariat can find unity, and with unity it can find strength. Strength enough for a revolution that is, and in many cases they were successful.
Proletariat in a capitalistic economy, as per the idea of Karl Marx and Friedrich Engels contained in Communist Manifesto, are the people who sell their labour for wages. They constitute the underprivileged class in the society, exploited and oppressed by bourgeoisie, who are the people who own and manage the means pf production.
It is in the interest of bourgeoisie to keep the proletariat under their dominance, so that they can continue to earn high profits at the cost of poverty and misery to the proletariat. In a condition like this, Marx and Engels believed, Proletariat can overthrow the oppressive dominance of bourgeoisie only through revolutionary. Proletariat are the only people to benefit from such a revolution, and therefore they are to only ones to be genuinely interested in staging a revolution.