إِنَّمَا الْأَعْمَالُ بِالنِّيَّاتِ، وَإِنَّمَا لِكُلِّ امْرِئٍ مَا نَوَى فَمَنْ كَانَتْ هِجْرَتُهُ إِلَى اللَّهِ وَرَسُولِهِ، فَهجْرَتُهُ إِلَى اللَّهِ وَرَسُولِهِ، وَمَنْ كَانَتْ هِجْرَتُهُ لِدُنْيَا يُصِيبُهَا أَوِ امْرَأَةٍ يَنْكِحُهَا، فَهِجْرَتُهُ إِلَى مَا هَاجَرَ إِلَيْه"Actions are (judged) by motives (niyyah), so each man will have what he intended. Thus, he whose migration (hijrah) was to Allah and His Messenger, his migration is to Allah and His Messenger; but he whose migration was for some worldly thing he might gain, or for a wife he might marry, his migration is to that for which he migrated." (Muslim and Bukhari)
Intention in Arabic is نية. Sincerity in every action we do is vital, because Islam does not allow us to carry out any deed irresponsibly. And hence, we should have sincerity in whatever we do. And this sincerity is called our intention.
Our actions depend on our intentions, as stated in the hadith above, and they will be judged based on their intentions. When a Muslim does anything, and his intention is to please Allah, then he will definitely get a reward for it, be it reading a good book or cleaning up his house.
Purifying our intentions is important. We should make sure that our intentions before doing something is pure and for something good. Not to take advantage of someone or something, or anything else bad. We should always have good intentions if we want reward for what we're doing. Renewing our intentions while we do anything can help remind us of our main goal, that is to please Allah, and in the end, gain ourselves some ajr (reward).
I ask Allah to purify our intentions for us, and help us please Him with all our might, in whatever we do, be it studying, cleaning up, or even having a conversations with a friend. Ameen. :)