Imala means pronouncing ا, bringing its pronunciation closer to an English sound made by the letter «a» in the article «a» , «an». This sign is placed under such alif «•».
وَقَالَ ٱرۡكَبُواْ فِيهَا بِسۡمِ ٱللَّهِ مَجۡرٜىٰهَا وَمُرۡسَىٰهَآ
Surah Hud, ayah 41
Note: in some publications this sign has the shape of a rhombus:
Also this sign «•» is placed above alif in the word ءَا۬عۡجَمِيّٞ in the 44 ayah of surah Fussilat (41 surah).
The presence of this sign here without a vowel instead of hamzah points out that hamzah is simplified (at-taskheel — التَّسْهِيلُ), that is, its pronunciation is something between ء and ا.