What is bold?

Bold is a heavy kind of writing. The characters are thicker than normal, as you see here:

Normal type and Bold type

This sentence is in normal type.
This sentence is in bold type.

The noun is "bold". The adjective is "bold". So we can also call bold "bold type".

When to use bold

We use bold for emphasis, when we want to make sure that our reader sees and takes in certain words. The main examples are:

It is better not to write entire paragraphs in bold. It is not easy to read a lot of words all together in bold (like this paragraph). So if you want readers to read your writing, better reserve bold for a few important words and phrases only.
Bold is not really possible in handwriting, but we can show emphasis with underlining (         ).