indoor bath; private bath
Refers to a bath inside a private home, as opposed to a public bath (外風呂) or sento. Often implies a bath used only by the family.
This apartment comes with a private bath.
Which do you prefer, an indoor bath or an outdoor bath?
内風呂 is an indoor bath, the direct opposite of the outdoor 露天風呂.
Compound of 内 (uchi, 'inside') and 風呂 (furo, 'bath'). The reading うちぶろ is a straightforward combination of the kun'yomi of 内 and the on'yomi of 風呂.