To subline something is to put it under a line of text to make it stand out.
Sublining can make important things easy to see.
Examples:
I will subline the title to make it stand out.
She sublines the important words in her notes.
The teacher sublines the key points in the lesson.
I subline the most important part of the story, like a secret message.
He sublines the whole page, so you can't miss it.
Subline the word 'important' so everyone can see it.