Each language belongs to a different community whose families originally came to Singapore from elsewhere in Asia. English is used at school and at work and helps everyone talk to one another. Mandarin, Malay and Tamil are spoken at home, at festivals, and with grandparents.
Malay is Singapore's 'national language' - the language of the country's anthem - because the area around Singapore is mostly Malay-speaking. The Singapore anthem, Majulah Singapura ('Onward Singapore'), is sung in Malay even by children whose families don't speak it at home.
Many Singaporeans also speak a friendly playful mix called Singlish. It is English sprinkled with words borrowed from Malay, Chinese and Tamil. If someone says 'Can lah!' it means 'Yes, of course!' - the 'lah' on the end is just for friendliness, like a smile.
Speaking more than one language is normal in most of the world. Singapore just happens to put all four right out there on the signs, so you can see them every day. It is a constant reminder that lots of cultures live here together.