The word lumpenproletariat doesn't occur in the 1888 authorized English translation; it is Marx's original German for what the authorized edition translates as the dangerous class, the social scum. The word literally means the rag-proletariat and refers to beggars, thieves, criminals, drifters, and bohemians.
Marx says of the lumpenproles: "The 'dangerous class,' the social scum, that passively rotting mass thrown off by the lowest layers of old society, may, here and there, be swept into the movement by a proletarian revolution, its conditions of life, however, prepare it far more for the part of a bribed tool of reactionary intrigue" (Section1.46).
In other words, the proletariat are the ones who revolt, and while the lumpenproles might join in, they're more likely to be bribed to serve the reactionaries (people who don't want a revolution).
The lumpenproles are quite interesting because many people tend to say Marx advocates for everyone who's oppressed—but this passage makes it clear that in the Manifesto, at least he sets aside a segment of the oppressed population as either largely irrelevant or as a (bribed) force against the proletariat.
You might wonder what communists, with their emphasis on wage-laborers uniting, make of infants or the very sick, who have no ability to work for wages at all, or those who do work or service for the community but are not paid in wages. Marx does say that under communism, "we shall have an association, in which the free development of each is the condition for the free development of all" (Section2.73), which suggests each individual is to be supported.
Perhaps his emphasis on wage-laborers was to unite workers into a large enough bloc to violently overthrow the rich and their armies. But might strengthening support for what we today call unpaid domestic labor, or for simple acts of reciprocal giving, provide another path out of rule by the rich?