Simple explanation: He goes there, I go there, he goes there, I go there, and so on.
It is just the ability to calculate your opponent's possible replies to your move and then your reply and again the possible replies and so on. In forced lines it may be relatively easy, as your opponent may be limited in the choice of replies, in some positions, it can be impossible to even go beyond the first reply as there may be many possibilities open.
Studying tactical problems will increase the ability to calculate the forced lines, studying positional problems will help in the 'quieter' situations. Experience will also be a great teacher, as some lessons aren't learnt until they are burnt into the memory by a bitter loss :)
I'm a high school student who's been playing for about a month. I've been hearing every now and then about 'calculation'. I've been trying to look it up but I can't find anything. Could someone explain to me what exactly it is and how I can learn to use it, or post a link?