Word order is a part of grammar. It has to do with the order words are in a sentence. The word order is often different between languages. For example, in English, people say "I only play tennis sometimes." In German, they would say "Ich spiele nur manchmal Tennis," which if they translate only the words says "I play only sometimes tennis." In Norwegian that same sentence would be "Jeg spiller bare tennis noen ganger", directly translated to "I play only tennis some times" in English. In Portuguese this sentence could be "Eu só jogo tênis algumas vezes"; translating each word to English: "I only play tennis some times". Or even in Portuguese people can change the word order to "Eu jogo tênis só algumas vezes" ("I play tennis only some times"), but they cannot say "Eu jogo só tênis algumas vezes", because this means "I play only tennis sometimes".
[change] Subject, Object and Verb
In English, a simple sentence with a verb (an action), subject (who or what is doing the action), and an object (who or what the action is done to) is written in a Subject-Verb-Object word order. For example, in the sentence "Robert opens the door", Robert is the subject, opens is the verb and door is the object. In other languages, sentences like this can be in different orders. For example, in Latin, that sentence could be written "Robert ianuam aperit", literally "Robert the door opens". It could even be written "aperit ianuam Robert". Languages that let you choose how to order the words often have a grammatical case system. In that sentence, "ianuam" is the accusative case of ianua (door). Accusative case means that the noun is the object of the sentence. "Robert" is in the nominative case, which means that it is the subject of the sentence. In English, changing the word order to "The door opens Robert" will change the meaning of the sentence. In Latin, however, "Robert ianuam aperit" and "ianuam Robert aperit" mean the same thing because ianuam is in the accusative case, so it is the object and Robert is the subject. Changing the cases of the words, however, to "Robertem ianua aperit" will change the meaning of the sentence - ianua is now in the nominative case so it is the subject and Robert is now the object.