Yay! An easy topic. Tempo is just the speed of the piece of music. You can take any piece of music and play it slow, play it fast or play it somewhere in between. Simple as that. Nothing changes but the speed at which the half notes, or quarter notes, or eight notes — whatever — go by in the time line. Tempo is measured in BPM, or 'Beats Per Minute', and often you'll see it written on sheet music or even tablature as = 90, meaning 90 quarter notes must be played every minute. Quarter notes are the most common unit to use. Just as we musicians can vary the key of a tune to suit the range of the singer, for example, we can also vary the tempo to suit our needs.
There are many examples of the same tune being performed and recorded at different tempos. Metronomes are used to measure tempo. There are a few different kinds, but they all do the same thing: emit a click or beep or flash a light at different tempos, measured as 'beats per minute', that you can play along to. Tempo need not be strict, you can speed up or slow down in a tune if that's the effect you want to convey, but generally speaking, tempo is consistent throughout a piece of music. The most common place to slow down is at the very end of a tune and it's known as a 'rall', short for Rallentando, Italian for 'slowing down'.