Something that is "unlikely" will probably not happen. These are some things that are "unlikely":

• a snowstorm in the summer
• getting a job that you're not really qualified for
• finding an earring that you dropped in a crowded place

When you use the phrase "highly unlikely", you mean that something is very unlikely, or it's almost sure not to happen. Here are some "highly unlikely" events:

• a snowstorm in the desert
• winning the lottery
• marrying a famous actor that you have a crush on

The word "highly" also means "very" in these other combinations:

highly sensitive

highly contagious

highly toxic

highly concentrated

highly annoying

But "highly" can't be used before most words in this way, so stick with the combinations above and ones that you hear or read other people using.