They basically mean the same thing. "Heard" is Past Tense. "Have heard" is Present Perfect tense. You use the Present Perfect to say that an action happened at an unspecified time before now. The exact time is not important. You Cannot use the Present Perfect with specific time expressions such as: yesterday, one year ago, last week, when I was a child, when I lived in Japan, at that moment, that day, one day, etc. You CAN use the Present Perfect with unspecific expressions such as: ever, never, once, many times, several times, before, so far, already, yet, etc.

Hello Jane, Using the present perfect tense "you have heard" is connected with the recent past and usually would mean that I have just said something a while ago or just now. It can't mean the thing is said in the far past ,whether yesterday , days ago or a year ago. In the later case the past tense is used. If time is not specified literally, like you don't mention when the hearing and saying actions took place using those two sentences refer to distinctive meanings: " you heard what i said". The actions are terminated in the past. "you have heard what I said" The actions just took place right now or a while ago connecting the near past with the present.