It means it has a very high concentration of sperm. It's the sperm that makes semen clumpy, and kind of yellowish. It's not always completely white as some people think. Also, if you haven't ejaculated in a while, it tends to be more yellowish than if you ejaculate everyday.
Yellow semen is fine. I should say yellowish, not very yellow. If its very yellow, yeah, it could be sign of an infection. Also brown semen is a sign of blood in it, and if it happens often, there is likely a problem with one of the testicles.