In a nutshell, remote sensing can be described like this: Gathering information about something without touching it. That is essentially what remote sensing is. Remotely sensing what a place is like.
That is a simple explanation. Here is more. There is active remote sensing, and passive remote sensing. Passive remote sensing is indirectly acquiring data about land via radiation given off by the land. Active remote sensing is acquiring land data by the emission of energy and detecting the features of the land.
Think of it like this. You have a remote control. You are remotely changing the television channels. Remote sensing, you are remotely sensing what the land is like. You are collecting land information at a distance.
To learn more, watch this video.