their meanings are the same but there is a rule about it in turkish it is about subject-verb harmony. if your subject is plural you need to use plural suffix for verb and ''onlar'' is one of them. so onlar yerler is the true one. for other personal pronouns: BEN yerim. SEN yersin. O yer. BİZ yeriz. SİZ yersiniz. Onlar yerler.

There is no difference in meaning, you can use both of them. If the subject is "human" and "plural", verb can be both singular and plural. However, as a Turkish native speaker i do not prefer to use plural one. It does not sound good.