Lemmy uses for its markup a language called "markdown". It’s the same one used on Reddit, Stack Exchange, as well as in a modified form on Discord.

Markdown requires two line breaks to define a new paragraph to allow you to break your paragraphs across multiple lines, a useful ability to have with writing that’s going to be tracked by Git, or displayed on some old-school text editors. It also gives you the ability to separate between two paragraphs
and one paragraph with a line break in it.
Which is a useful thing to be able to do in some circumstances, like when writing poetry, or sharing your results in !dailygames@lemmy.zip. To enter a "line break", end a line with two spaces and then press enter only once.