Certainly, We have made this easy in your tongue, in order that they may remember.
means, `We have made this Qur'an, which We have sent down, easy, plain and clear, in your language which is the most eloquent, clear and beautiful of all languages.'
...
لَعَلَّهُمْ يَتَذَكَّرُونَ
in order that they may remember.
means, in order that they may understand and know. Despite the fact that it is so plain and clear, there are still people who disbelieve, who stubbornly go against it. Allah says to His Messenger, consoling him and promising him victory, and warning those who reject him that they will be destroyed.
فَارْتَقِبْ إِنَّهُم مُّرْتَقِبُونَ