Poster from the Singapore Crime Prevention Council:
Here's a government website on this subject.
The ad is given in English and Chinese versions. Someone who knows only Modern Standard Mandarin (MSM) would not be able to understand the key portion of the Chinese version of the warning nor would British and American speakers be able to understand the "Ah Longs" of the English version — even with the help of the visual clues.
"Ah Long" is one of those special Singaporean terms that are not familiar to speakers of English outside of the Lion City and adjoining parts of Malaysia. We may note that the corresponding term in Chinese is dàěrlóng 大耳窿 (lit., "big ear hole / cavity"). "Ah Long" and dàěrlóng 大耳窿 both mean "loan shark".
This site and others claim that Ah Long entered Singaporean English from Hokkien. That may well be, but Ah Long itself derives from Cantonese, since the Hokkien word for "ear hole" is hi"-khang (POJ Romanization), which would be better written in characters as 耳穴 or 耳孔, not 耳窿. Here is the same term (Romanized as hī-khang) in an old dictionary of Amoyese which gives many colorful expressions employing it.
The supposed etymology of daai6 ji5 lung4*1 大耳窿 ("big ear hole") is so far-fetched and downright weird that I don't want to spend time repeating it here, but those who are interested in pursuing it may turn to these two sites.
The prevalence and tactics of the Ah Longs in Singapore and Malaysia are described in this article. There's even a scary C-grade Cantonese movie (with several well-known Hong Kong stars and a Malaysian director!) on loan sharks (link, link). And here's a clip if you want to get an idea of why the Ah Longs are so feared.
Here's a funny juxtaposition of two banners, with someone perceived to be an Ah Long greeting residents of a housing complex near the Ang Mo Kio MRT Station in Singapore on the top banner and an injunction against dealing with Ah Longs on the bottom banner.
A final note: loan words from Chinese into Singaporean English can take the plural ending.
[Thanks to Janet Williams, Grace Wu, Chia-hui Lu, Sophie Wei, Melvin Lee, Leander Seah, and David Branner]