وَتَثْبِيتًا مِّنْ أَنفُسِهِمْ
(while they in their own selves are sure and certain) meaning, they are certain that Allah shall reward them for these righteous acts with the best rewards. Similarly, in a Hadith collected by Al-Bukhari and Muslim, the Messenger of Allah ﷺ said,
«مَنْ صَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا»
(Whoever fasts Ramadan with faith and expectation...) meaning, believing that Allah commanded the fast, all the while awaiting His reward for fasting it.
Allah's statement,
كَمَثَلِ جَنَّةٍ بِرَبْوَةٍ
(is that of a garden on a Rabwah) means, the example of a garden on `a height above the ground', as the majority of scholars have stated. Ibn `Abbas and Ad-Dahhak added that it also has flowing rivers.
Allah's statement,
أَصَابَهَا وَابِلٌ
(Wabil falls on it) means, heavy rain as we stated, So it produces its,
أُكُلُهَا
(yield of harvest) meaning, fruits or produce,
ضِعْفَيْنِ
(doubles), as compared to other gardens.
فَإِن لَّمْ يُصِبْهَا وَابِلٌ فَطَلٌّ
(And if it does not receive Wabil, a Tall suffices it.)
Ad-Dahhak said that the `Tall' is light rain. The Ayah indicates that the garden on the Rabwah is always fertile, for if heavy rain does not fall on it, light rain will suffice for it. Such is the case regarding the believer's good deeds, for they never become barren. Rather, Allah accepts the believer's righteous deeds and increases them, each according to his deeds. This is why Allah said next,
وَاللَّهُ بِمَا تَعْمَلُونَ بَصِيرٌ
(And Allah is All-Seer of what you do) meaning, none of His servants' deeds ever escapes His perfect watch.
Tafsir Ibn Kathir (abridged, English).