Visa run is a brief trip over the international border to purposefully reset the visa validity of the originating country. This is very common between Singapore and Malaysia, and the timer resets when you exit one country and entry another. When leaving Singapore, the popular option is to leave via Singapore-Johor Causeway, spend a day to enjoy some shopping in Johar Bahru or see Legoland Malaysia, and again go back to Singapore for another 30/90 days.
If a foreigner has stayed for 30 days already and tries to do a visa return (or U-turn), there are 50/50 chance that he’ll be questioned on the purpose of Singapore stay. It is possible that he’ll be asked for an interview at the border and asked questions like where will you accommodate? When do you leave as per your plan? What are you doing in Singapore? Where do you work and from where did you get such money for a long vacation?
Some foreigners might not be stopped at first U-turn, but the chances increase with every entry, and it’ll be practically 100% after 4-5 times that questions will be asked.
Then it’ll be a game of answers, but it’s hard to satisfy them or prove that you are an addicted tourist to Singapore if you have spent 3-4 months here.
In case you fail to convince the officer that you are a tourist, the officer will refuse your entry (no matter even if you have a valid visa).