Hints at the input fields make the form more user friendly. They help explain to people how to fill in the data and why to fill it in.
Probably, you often see a solution where the hint is hidden behind a question mark. When you click it, it opens a popover, and you can read the hint in it.
There are two problems here. First, it is not there if something is hidden, which means the user is likely to miss or not notice. Secondly, it's always an inconvenient solution; the small icon is hard to get to. The popover is always off-screen and disappears when accidentally tapped on the page or inside the popover.
Do not be afraid of hints with a long text. They can be shown at once, and it won't complicate the form in any way and won't scare the user. On the contrary, it will help to understand faster and easier what data are required and how to fill them out.
It's a bad decision to place the hint text inside the label name of input field. It reduces the form's readability.
A good way to place hints below the label, below the input, or to the right of the input field, if space allows it.