Learn Portuguese (Portugal): Eles varrem o chão e lavam-no.

When a verb ends in a nasal sound—like the -m in lavam—the direct object pronouns o, a, os, and as gain an n to become no, na, nos, and nas. Because lavam ends in a nasal -m, the pronoun o (meaning "it") transforms into -no to make the words flow together naturally.

What exactly does the -no stand for in this sentence?

It stands for o chão (the floor). Because chão is a masculine singular noun, we replace it with the masculine singular pronoun o ("it"). When that o attaches to the nasal ending of lavam, it becomes -no.

Why doesn't the first verb (varrem) have a pronoun attached?

The first verb, varrem (sweep), is followed directly by the noun it affects: o chão (the floor). We only use the pronoun -no on the second verb, lavam (wash), so we don't have to awkwardly repeat "the floor" a second time.

Does adding -no change where the spoken stress falls on ?