ます form

Ma-su (ます) form is a formal way of communicating in Japanese. To sound mature and educated use this form when speaking to other people you don’t know.

1. Example

わたしは・さかなお・たべます。

Above sentence is a formal way of saying “I eat fish”. While saying:

わたしは・さかなお・たべる。

is an informal and maybe even immature way of saying the same sentence.

2. Important Distinction

Formal form ≠ Plain Form = Dictionary Form

Source