"Buy" (aka 'Long') is investing in a security hoping for the price to go upwards.
'Short' is investing in a security hoping for the price to go downwards.
If there is a security that you think will go up in price then you buy and you make profit as the price rises.
If there is a security that you think will go down in price then you short and you make profit as the price drops.
When you want to get rid of the shares you have longed (buy) then you sell them.
When you want to get rid of the shares that you have shorted then you 'cover' them.