We talk about “trains of thought.” You can think of these as being like real locomotive trains that pull into a busy station and then go rattling off. Most of them don’t go anywhere that we particularly want to go (most of them are to do with worrying, getting angry, running ourselves down, etc). But our mind is like a little kid that’s very restless and curious, and keeps going through the open doors into the carriages.
Before we know it we’re miles away from where we wanted to be (in dangerous territory, often!), and it takes us forever to get home.
By learning meditation you can learn just to watch the trains pulling up and pulling away, being aware of them and choosing not to get into them.
Are there any trains we want to get into? Yes. Some thoughts can be useful, if they are reflections about our meditation, for example. Such thoughts take us deeper into our meditation.
One difference between useful thought trains and those that take us into distractions, is that when we’re reflecting (as opposed to being distracted), we know what we’re thinking and why, and what effect those thoughts are having). By contrast distracted thoughts are like dreams — we don’t know we’re in them until we “wake up.”
But it can take a while to recognize which thoughts are useful and at first it’s not a bad idea simply to treat all thoughts as distractions and to let them all depart from the station of the mind while you “just sit” on the platform.