Although the Sun doesn't seem to change from day to day, upon closer inspection, it is a very active object. One of the ways of measuring this Solar Activity is by counting the number of sunspots that can be seen from day to day (see centre of image).
The amount of Solar activity slowly changes from year to year and reaches a maximum about every 11 years. It is not known why this cycle exists, but if you want to have a look for yourself you can try our Sunspots Workshop, which has measurements for the last 100 years.
In addition to sunspots, the unwinding of twisted magnetic lines within the Sun can lead to very energetic events known as solar flares, which can see large amounts of matter flung out into the Solar System. If the flare was unfortunate enough to head towards Earth, there is a danger that it could do a lot of damage to satellites in Earth orbit.