I think the doc has asked you for a semen analysis
because your wife is not able to conceive.Well ,you may not be having a disease. The semen is your body secretion from your male reproductive organs which you ejaculate at the time of intercourse or during masturbatioin,.A semen analysis will tell you about the nature of your semen, and the the contents within it, most importantly spermatozoa
.Spermatozoa are your male gametes which are required for reproduction.Their count, motility ,morphology etc are important parameters to determine a future pregnancy
. Apart from that, the analysis will give other info about your semen also.
Its is a basic test done in couples who are keen to conceive. so don't worry and go ahead.all the best