Activities track down what you have done, when and for which customer: it's the most important thing, since starting from them you can recreate the project history.
On the left you can find tracker fields (worked hours are calculated automatically, you don't have to insert them), while on the left there is a very useful tool: a timer
I don't know about you, but every time I start working... well I forgot to write down the time. So it happens that I don't exactly know how much I worked, and that's bad, really bad.
When you start working, trigger the timer, then go back on your project. When you have finished, stop it. It will automatically compute the start/end time and it insert them into the fields.