COUNTABLE. What you CAN count. One, two, three, four...
One - singular.
One table, one armchair, one rocking chair, one sofa.
A table, an armchair, a rocking chair, a sofa.
Two or more - plural. Two chairs. Three sofas.
UNCOUNTABLE. What you CANNOT count. There's no "one", no two, no plural ending.
Furniture = all of those items together, all the pieces of furniture
That is nice furniture. Is that expensive furniture? Was it expensive? That furniture is for dolls.
Furniture = kalustoa, huonekaluja
A piece of furniture = huonekalu