What do yellow leaves mean on a houseplant?

That depends on what plants you have.For the correct answer to be given

could be most anything -- depending on the KIND of plant -- this site can advise you http://ask.extension.org --- but it's not 'happy' -- so don't give fertilizer unless you KNOW that is the issue -- never 'feed' a sick plant

It depends upon the individual plant. Google care for ______ (insert the type of plant) and you'll come up with many sites that will help you. Plants need water, light, good soil and warmth. Some also just shed leaves as they grow.

Can You Take A Picture With Your Phone Of The Plant And Show It To Your Local Nusery. That Is Also Another Option.

Could be too much water, not enough water, too drafty, too much light, not enough light, moving it too often, too humid, not humid enough, lack of minerals in the soil, too much fertilizer, not enough fertilizer