drain in floor next to washing machine

I have a drain on the floor next to my washing machine. What is the purpose of this drain? I assume to handle any water if the machine overflows? I ask because last week it was giving off a bad smell. By suggestion of someone who knows a little about plumbing, I poured some fresh water down there and the smell went away. The water flowed freely, so I guess there's no blockage. When I look down this pipe, I can see water sitting there. Is there supposed to be water sitting in there? Again the fresh water was forcibly poured down with no problem. I think it's starting to give off a little smell again. I don't know if I need to call a plumber or not. Thanks for any info you can give me.

That floor drain sounds like it has a trap. The trap is supposed to have water and if there is no trap primer then you would have to occassionally put water in it to keep sewer gases out.

You identified the purpose of the drain correctly. The problem that frequently occurs with floor drains is that since they a seldom used, the water in the P trap under them evaporates and this allows sewer gas to escape. The easiest solution is to do as you did, periodically pour some water into it to refill the trap.