Help! I am getting the following error message on my MacBook Air "Your hard disk does not have enough space. What do I do?

I have a Macbook Air that I bought in 2011. I am using iOS X Yosemite 10.10.5 and the processor is 1,8 GHz Intel Core i5

Over the past week or so I have occasionally received a pop up that says "Your hard disk does not have enough space".

I am very tech-ignorant but managed to locate the storage of the HD which indicates that there is 8.79 GB free out of 120 GB. The color bar shows 75 GB taken up by "other". What does that mean? (Photos are 15 GB; Apps are 14GB; Movies and Audio are indicated at much less)

How do I make space? What will happen if I do not?

Thank you for any help!