That is the cache indicator. Blue = disk cache, green = RAM cache. Anyway, this is basic stuff required to understand how AE works and you need to read up.
Why would you want to delete them?
If you clear the cache, they'll go away, but if you want to play your timeline, you'll need them.
As Mylenium says, this is bare-bones basics and you are only going to be confused and frustrated if you try to work with AE as you are. I highly recommend going through the resources that he linked for you.
You can turn off the cache indicators in the options for the timeline. You get there by clicking on the little icon in the top right corner of the timeline. With some configurations of AE these indicators may cause problems. What is your current AE Version down to the last decimal point and what is your OS?